Electrolyte emulsion and process for producing same

ABSTRACT

A method for producing an electrolyte emulsion, the method including: Step (1) in which an ethylenic fluoromonomer and a fluorovinyl compound having an SO 2 Z 1  group, wherein Z 1  is a halogen element, are copolymerized at a polymerization temperature of 0° C. or higher and 40° C. or lower to provide a precursor emulsion containing a fluoropolymer electrolyte precursor; and Step (2) in which a basic reactive liquid is added to the precursor emulsion and the fluoropolymer electrolyte precursor is chemically treated, whereby an electrolyte emulsion with a fluoropolymer electrolyte dispersed therein is provided, wherein the electrolyte emulsion has an equivalent weight (EW) of 250 or more and 700 or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No. 14/822,158filed Aug. 10, 2015, which is a divisional application of U.S.application Ser. No. 13/496,996 filed Mar. 27, 2012, which is a NationalStage of International Application No. PCT/JP2010/066227 filed Sep. 17,2010, claiming priority to Japanese Patent Application No. 2009-217693filed Sep. 18, 2009, the contents of all of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an electrolyte emulsion which issuitable for, for example, electrolyte membranes for polymer electrolytefuel cells, and a method for producing the same.

BACKGROUND ART

Fuel cells are cells directly converting chemical energy of fuels intoelectric energy by electrochemically oxidizing hydrogen, methanol, andthe like in cells, and then extracting the electric energy. They aretherefore focused on as clean electric energy sources. In particular,polymer electrolyte fuel cells can drive at temperatures lower thanother cells, and they are expected as alternative power sources forautomobiles, household cogeneration systems, portable electricgenerators, and the like.

Such a polymer electrolyte fuel cell comprises at least a membraneelectrode assembly. The membrane electrode assembly comprises anelectrolyte membrane and gas diffusion electrodes. Each gas diffusionelectrode is formed by stacking an electrode catalyst layer and a gasdiffusion layer, and the gas diffusion electrodes are joined to therespective faces of the electrolyte membrane. The electrolyte membraneherein is a material having a strong acid group such as a sulfonic acidgroup and a carboxylic acid group in a polymer chain, and havingproton-selective permeability. Examples of such an electrolyte membraneinclude perfluoro proton exchange membranes typically such as Nafion(registered trademark, Du Pont) having high chemical stability.

In order to drive a fuel cell, a fuel (e.g. hydrogen) is supplied to agas diffusion electrode on the anode side, while an oxidant (e.g. oxygenor air) is supplied to a gas diffusion electrode on the cathode side,and both of the electrodes are coupled through an outside circuit.Thereby, the fuel cell operates. Specifically, in the case that hydrogenis used as the fuel, hydrogen is oxidized and generates a proton on ananode catalyst. This proton passes through an electrolyte binder in theanode catalyst layer, moves inside the electrolyte membrane, and reacheson a cathode catalyst through the electrolyte binder inside the cathodecatalyst layer. On the other hand, an electron generated at the sametime of the proton by oxidation of hydrogen reaches the gas diffusionelectrode on the cathode side through the outside circuit. On thecathode catalyst, the proton and oxygen in the oxidant react to generatewater. At this time, electric energy is generated.

Since polymer electrolyte fuel cells show a high energy conversion ratewith a small environmental burden, they are expected as stationarycogeneration systems and vehicle-mounted power sources. In theautomobile applications, fuel cells are generally driven at around 80°C. at the present time. In order to popularize fuel-cell vehicles,however, downsizing of radiators and simplification of humidifiers, andresulting cost reduction are required. For this purpose, an electrolytemembrane is demanded which is capable of being applied to driving underhigh-temperature and low-humidity conditions (corresponding to a drivingtemperature of 100° C. to 120° C. and a humidity of 20 to 50% RH), andwhich shows high performance under wide driving environments (roomtemperature to 120° C./20 to 100% RH). Specifically, as shown inNon-Patent Document 1, the proton conductivity is required to be 0.10S/cm or higher at 50% RH for a driving temperature of 100° C., and theproton conductivity is required to be 0.10 S/cm or higher at 20% RH fora driving temperature of 120° C.

The conductivity of a conventional perfluoro proton exchange membrane,however, greatly depends on humidity, and it greatly decreasesparticularly at 50% RH or lower. Patent Documents 1 to 3 disclose afluoroelectrolyte membrane having an equivalent weight (EW), that is, EW(g/eq) which is a dried weight per equivalent of a proton exchangegroup, of 670 to 776. As mentioned here, a reduction in the EW value, inother words, an increase in the capacity of the proton exchange leads toan increase in the conductivity. Further, Patent Document 4 discloses anelectrolyte membrane which is less likely to be hydrothermally dissolvedeven at a low EW, and exemplifies an electrolyte membrane with an EW of698. Patent Document 5 discloses one example of a method for producing apolymer electrolyte with an EW of 564.

In addition, perfluoro proton exchange membranes are known todeteriorate due to long-term use, and various stabilizing methods areproposed. For example, Patent Document 6 discloses a fluoropolymerelectrolyte obtained through a polymerization step in which materialsare copolymerized at a polymerization temperature of 0° C. to 35° C.using a radical polymerization initiator that comprises a fluorocompoundhaving a molecular weight of 450 or higher.

Patent Document 1: JP 06-322034 A

Patent Document 2: JP 04-366137 A

Patent Document 3: WO 2002/096983

Patent Document 4: JP 2002-352819 A

Patent Document 5: JP 63-297406 A

Patent Document 6: JP 2006-173098 A

Non-Patent Document 1: H. Gasteiger and M. Mathias, In Proton ConductingMembrane Fuel Cells, P V 2002-31, pp. 1-22, The Electrochemical SocietyProceedings Series (2002)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, each of the electrolyte membranes disclosed in Patent Documents1 to 6 still has a low conductivity at 50% RH or lower; the conductivityis far from 0.10 S/cm.

The present inventors have found that use of a fluoroelectrolyteprecursor polymerized by a unique method enables to control anion-cluster structure formed in a fluoroelectrolyte, and that control ofthe ion-cluster structure in the electrolyte membrane causes highconductivity even at low humidity. The technique achieved by the presentinventors provides an electrolyte having high conductivity even underhigh-temperature and low-humidity conditions, and thereby enables toprovide a fuel cell with higher performance.

On the other hand, the present inventors have also found that problemsin processability occur; for example, a fluoropolymer electrolyte iseasily softened if it has a low equivalent weight, and a membraneproduced therefrom absorbs moisture in the air to suffer creases.

Means for Solving the Problems

Thus, the present invention aims to improve processability of afluoropolymer electrolyte having a low equivalent weight and high protonconductivity.

In other words, the present invention provides a fluoropolymerelectrolyte material whose processability is improved and which is easyto produce by preparing a spherical particulate substance having a largeparticle size as a fluoropolymer electrolyte and making an emulsion inwhich the particulate substance is dispersed in an aqueous medium.

The present invention relates to an electrolyte emulsion comprising anaqueous medium and a fluoropolymer electrolyte dispersed in the aqueousmedium, wherein the fluoropolymer electrolyte has a monomer unit thathas an SO₃Z group (wherein Z is an alkali metal, an alkaline-earthmetal, hydrogen, or NR¹R²R³R⁴, wherein R¹, R², R³, and R⁴ each areindividually a C1-C3 alkyl group or hydrogen), the electrolyte has anequivalent weight (EW) of 250 or more and 700 or less, a protonconductivity at 110° C. and relative humidity 50% RH of 0.10 S/cm orhigher, and the ratio (the number of SO₂F groups)/(the number of SO₃Zgroups) of 0 to 0.01, and the electrolyte is a spherical particulatesubstance having an average particle size of 10 to 500 nm.

The equivalent weight (EW) of the fluoropolymer electrolyte ispreferably 250 or more and 650 or less.

Preferably, the fluoropolymer electrolyte comprises a repeating unit (α)derived from an SO₃Z group-containing monomer represented by thefollowing formula (I):

CF₂═CF(CF₂)_(k)—O₁—(CF₂CFY¹—O)_(n)—(CFY²)_(m)-A¹  (I)

wherein Y¹ is F, Cl, or a perfluoroalkyl group; k is an integer of 0 to2; 1 is 0 or 1; n is an integer of 0 to 8; n Y¹s may be the same as ordifferent from each other; Y² is F or Cl; m is an integer of 0 to 6,provided that if m=0, 1=0 and n=0; m Y²s may be the same as or differentfrom each other; A¹ is SO₃Z, wherein Z is an alkali metal, analkaline-earth metal, hydrogen, or NR¹R²R³R⁴ wherein R¹, R², R³, and R⁴each are individually a C1-C3 alkyl group or hydrogen; and a repeatingunit (β) derived from an ethylenic fluoromonomer that is different fromthe monomer that provides the repeating unit (a), and the repeating unit(a) is in an amount of 10 to 95 mol %, the repeating unit (β) is in anamount of 5 to 90 mol %, and the sum of the amounts of the repeatingunit (a) and the repeating unit (β) is 95 to 100 mol %.

In the fluoropolymer electrolyte, preferably, k is 0, l is 1, Y¹ is F, nis 0 or 1, Y² is F, m is 2 or 4, and A¹ is SO₃H.

In the fluoropolymer electrolyte, preferably, n is 0 and m is 2.

The fluoropolymer electrolyte preferably has a distance between ionicclusters of 0.1 nm or higher and 2.6 nm or lower at 25° C. and relativehumidity 50% RH by small angle X-ray measurement based on the followingformula (1):

d=λ/2/sin(θm)  (1)

wherein d is a distance between ionic clusters, λ is an incident X-raywavelength used in the small angle X-ray measurement, and θm is a Braggangle which indicates a peak.

The fluoropolymer electrolyte is preferably obtainable by chemicallytreating a fluoropolymer electrolyte precursor, and preferably, thefluoropolymer electrolyte precursor has a group that to be converted, bythe chemical treatment, into SO₃Z (wherein Z is an alkali metal, analkaline-earth metal, hydrogen, or NR¹R²R³R⁴ wherein R¹, R², R³, and R⁴each are individually a C1-C3 alkyl group or hydrogen), ismelt-flowable, and has a melt-flow rate of 0.01 to 100 g/10 min.

The chemical treatment is preferably a treatment of making thefluoropolymer electrolyte precursor in contact with a basic reactiveliquid.

The electrolyte emulsion preferably contains 2 to 80% by mass of thefluoropolymer electrolyte.

The aqueous medium preferably has a water content of more than 50% bymass.

The present invention also relates to a method for producing anelectrolyte membrane. The method comprises: applying the electrolyteemulsion to a substrate; drying the electrolyte emulsion applied to thesubstrate to provide an electrolyte membrane; and peeling theelectrolyte membrane off from the substrate.

The present invention also relates to an electrolyte membrane obtainableby the above method for producing an electrolyte membrane.

The present invention also relates to a method for producing anelectrode catalyst layer. The method comprises: dispersing compositeparticles of a catalyst metal and a conductive agent in the electrolyteemulsion to prepare an electrode catalyst composition; applying theelectrode catalyst composition to a substrate; and drying the electrodecatalyst composition applied to the substrate to provide an electrodecatalyst layer.

The present invention also relates to an electrode catalyst layerobtainable by the above method for producing an electrode catalystlayer.

The present invention also relates to a membrane electrode assemblycomprising the electrolyte membrane.

The present invention also relates to a membrane electrode assemblycomprising the electrode catalyst layer.

The present invention also relates to a polymer electrolyte fuel cellcomprising the membrane electrode assembly.

The present invention also relates to a method for producing anelectrolyte emulsion. The method comprises: Step (1) in which anethylenic fluoromonomer and a fluorovinyl compound having an SO₂Z¹ group(Z¹ is a halogen element) are copolymerized at a polymerizationtemperature of 0° C. or higher and 40° C. or lower to provide aprecursor emulsion containing a fluoropolymer electrolyte precursor; andStep (2) in which a basic reactive liquid is added to the precursoremulsion and the fluoropolymer electrolyte precursor is chemicallytreated, whereby an electrolyte emulsion with a fluoropolymerelectrolyte dispersed therein is provided. In the method, theelectrolyte emulsion has an equivalent weight (EW) of 250 or more and700 or less.

The above method for producing an electrolyte emulsion is preferably amethod for producing the aforementioned electrolyte emulsion of thepresent invention.

Effects of the Invention

Since the electrolyte emulsion of the present invention has the abovefeature(s), it shows good processability when processed into anelectrolyte membrane or an electrode catalyst layer, for example, andsuch products are easily produced. Thus, use of the electrolyte emulsionof the present invention enables to produce high-output fuel cells atlow cost and high productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the results of the examples and comparativeexamples, wherein the horizontal axis indicates a distance between ionicclusters and the vertical axis indicates 50% RH conductivity.

MODES FOR CARRYING OUT THE INVENTION

The following will describe modes for carrying out the present invention(hereinafter, referred to as embodiments of the invention) in detail.Note that the present invention is not limited to the followingembodiments and may be varied within the scope of its features.

The present invention relates to an electrolyte emulsion comprising anaqueous medium and a fluoropolymer electrolyte dispersed in the aqueousmedium.

The fluoropolymer electrolyte is a spherical particulate substancehaving an average particle size of 10 to 500 nm.

The term “spherical particulate substance” herein means a substantiallyspherical particulate substance, and the phrase “substantiallyspherical” means that the aspect ratio is 3.0 or lower. Generally, thecloser to 1.0 the aspect ratio is, the closer to a sphere the substanceis.

The aspect ratio of the spherical particulate substance is preferably3.0 or lower. The upper limit thereof is more preferably 2.0, andfurther preferably 1.5. The lower limit of the aspect ratio of thespherical particulate substance is, for example, 1.0.

If the shapes of the polymer particles show anisotropy, the emulsion islikely to have high viscosity, in general. In contrast, if thefluoropolymer electrolyte is a spherical particulate substance, theviscosity of the electrolyte emulsion is lower than that in the casethat the particles are not spherical, and the fluoropolymer electrolytedoes not affect handleability even though its solids concentration ismade high. Thus, for example, high productivity can be achieved in thecase of producing a membrane by a method such as cast film formation.

The average particle size of the fluoropolymer electrolyte is 10 to 500nm. If the average particle size is less than 10 nm, the active point iscovered and good cell characteristics may not be achieved in the casethat the electrolyte is used as an electrode material.

The upper limit of the average particle size may be 500 nm from theviewpoints of stability of the electrolyte emulsion and easy productionof the precursor emulsion. Even if the average particle size is higherthan 500 nm, the cell characteristics are not greatly affected.

As the average particle size is within the above appropriate range, theviscosity of the electrolyte emulsion is low, handleability is excellenteven though the solids concentration of the fluoropolymer electrolyte ishigh, and high producibility is achieved upon producing a membrane.

The average particle size of the fluoropolymer electrolyte is morepreferably 10 to 300 nm. The lower limit of the average particle size isfurther preferably 30 nm, and the upper limit thereof is furtherpreferably 160 nm.

The aspect ratio and the average particle size are determined asfollows. The electrolyte emulsion is applied to a glass substrate; theaqueous medium is removed to provide an aggregate of the fluoropolymerelectrolyte; the aggregate is observed using a scanning or transmissionelectron microscope, an atomic force microscope, or the like; 20 or moreparticles in the obtained image each were measured for the lengths ofits major and minor axes; the average value of the ratios of the lengthsof the major and minor axes (major axis/minor axis) is treated as theaspect ratio mentioned above, while the average value of the lengths ofthe major and minor axes is treated as the average particle sizementioned below.

The equivalent weight (EW), that is, the dried weight per equivalent ofan ion-exchange group, of the fluoropolymer electrolyte is 250 or moreand 700 or less. Since the EW is small as mentioned here, a high ionconductivity can be achieved, and thus the electrolyte is suitably usedfor producing an electrolyte material used for, for example, fuel cells.The upper limit of the EW is preferably 650. An EW of higher than 650may cause poor membrane producibility. The upper limit thereof is morepreferably 600, further preferably 550, and particularly preferably 500.The upper limit of the EW is also preferably 450. An EW of 450 or lowerleads to an extremely high ion conductivity, and thus the electrolyte isparticularly suitable as materials for producing electrolyte membranesand electrode catalyst layers used for fuel cells. Therefore, theelectrolyte can be particularly suitably used for various applicationsrequiring a high ion conductivity. The lower limit of the EW ispreferably 300, more preferably 350, and further preferably 390. Asmaller EW is preferable because it leads to a higher conductivity,while it may cause a higher solubility in hot water. Thus, the EW ispreferably within the above appropriate range.

Further, the fluoropolymer electrolyte has a proton conductivity of 0.10S/cm or higher at 110° C. and relative humidity 50% RH. Preferably, theproton conductivity at 40% RH is 0.10 S/cm or higher; more preferably,the proton conductivity at 30% RH is 0.10 S/cm or higher; furtherpreferably, the proton conductivity at 25% RH is 0.10 S/cm or higher;and particularly preferably, the proton conductivity at 23% RH is 0.10S/cm or higher. The proton conductivity of the fluoropolymer electrolyteis preferably as high as possible; for example, the proton conductivityat 110° C. and relative humidity 50% RH may be 1.0 S/cm or lower.

Since the fluoropolymer electrolyte has a proton conductivity in theaforementioned range, the electrolyte emulsion of the present inventionenables to produce products such as electrolyte membranes and electrodecatalyst layers which can be applied to driving under high-temperatureand low-humidity conditions and which show high performance in variousdriving environments.

The proton conductivity can be measured as follows using a polymermembrane water content test apparatus (e.g. an MSB-AD-V-FC polymermembrane water content test apparatus, BEL Japan, Inc.): a polymerelectrolyte membrane produced with a thickness of 50 μm is cut out intoa size of 1 cm in width and 3 cm in length, and is mounted on a cell forconductivity measurement; the cell for conductivity measurement is thenplaced inside a chamber of the test device, and the conditions insidethe chamber are adjusted to 110° C. and lower than 1% RH; water vaporgenerated from ion-exchange water is introduced into the chamber; theinside of the chamber is humidified to 10% RH, 30% RH, 50% RH, 70% RH,90% RH, and 95% RH in this order, and the conductivities at therespective humidities are measured.

Further, the fluoropolymer electrolyte has a unique ion-clusterstructure. In other words, the fluoropolymer electrolyte preferably hasa distance between ionic clusters of 0.1 nm or longer and 2.6 nm orshorter at 25° C. and 50% RH. FIG. 1 is a graph showing the results ofthe examples and comparative examples mentioned below, wherein thehorizontal axis indicates a distance between ionic clusters and thevertical axis indicates conductivity under high-temperature andlow-humidity conditions. The graph shows that the conductivity greatlyincreases as the distance between ionic clusters decreases to 2.6 nm orlower.

The upper limit of the distance between ionic clusters is morepreferably 2.5 nm. The lower limit of the distance between ionicclusters may be 0.5 nm, 1.0 nm, or 2.0 nm, for example.

The fluoropolymer electrolyte having a distance between ionic clusterswithin the above range enables to produce products such as electrolytemembranes and electrode catalyst layers which are particularly suitablefor driving under high-temperature and low-humidity conditions and whichshow high performance under various driving environments.

An ion cluster is an ion channel formed by aggregation of protonexchange groups, and perfluoro proton exchange membranes such as Nafionare considered to have such an ion-cluster structure (for example, seeGierke. T. D., Munn. G. E., and Wilson. F. C., J. Polymer Sci. PolymerPhys, 1981, 19, 1687).

The distance between ionic clusters d can be measured and calculated bythe following method.

The prepared fluoropolymer electrolyte is subjected to small-angle X-rayscattering measurement at 25° C. and 50% RH. The obtained scatteringintensities are plotted with respect to Bragg angles θ, and the Braggangle θm at a peak position derived from a cluster structure which isgenerally observed at 2θ>1°. The distance between ionic clusters d iscalculated by the following formula (1) based on the angle θm:

d=λ/2/sin(θm)  (1)

wherein λ is an incident X-ray wavelength.

In the case that the membrane for this measurement is produced by acasting method, the membrane is previously annealed at 160° C. Inaddition, the fluoropolymer electrolyte is treated so that the terminalgroup represented by an SO₃Z group is converted into SO₃H. The samplemembrane is left standing at 25° C. and 50% RH for 30 minutes or longerbefore the measurement, and then the measurement is performed.

In the fluoropolymer electrolyte, the distance between ion clusters isshort. Thus, protons presumably easily move between the ion clusters,and the electrolyte has high conductivity even at low humidity.

The fluoropolymer electrolyte has a monomer unit having an SO₃Z group (Zis an alkali metal, an alkaline-earth metal, hydrogen, or NR¹R²R³R⁴wherein R¹, R², R³, and R⁴ each are individually a C1-C3 alkyl group orhydrogen).

The fluoropolymer electrolyte has a ratio (the number of SO₂Fgroups)/(the number of SO₃Z groups) of 0 to 0.01. The ratio “(the numberof SO₂F groups)/(the number of SO₃Z groups)” is a ratio of the number ofSO₂F groups to the number of SO₃Z groups in the fluoropolymerelectrolyte, and the ratio can be determined as follows.

In the case of producing the fluoropolymer electrolyte by converting—SO₂F groups in a fluoropolymer electrolyte precursor having —SO₂Fgroups, for example, (the number of SO₃Z groups in fluoropolymerelectrolyte) can be approximately considered as {(the number of SO₂Fgroups in fluoropolymer electrolyte precursor)−(the number of SO₂Fgroups in fluoropolymer electrolyte)}. In other words, the ratio (thenumber of SO₂F groups)/(the number of SO₃Z groups) of the fluoropolymerelectrolyte can be determined by measuring (the number of SO₂F groups influoropolymer electrolyte) and (the number of SO₂F groups influoropolymer electrolyte precursor) by infrared absorptionspectrometry, and calculating the ratio therebetween.

Specifically, a film of the precursor prepared by a technique such asheat-pressing is subjected to Fourier transform infrared spectroscopy.The intensity (I_(0C)) at an absorption peak derived from CF₂ at around2364 cm⁻¹ and the intensity (I_(0S)) at an absorption peak derived fromSO₂F groups at around 2704 cm⁻¹ are measured, and the ratioA₀=I_(0S)/I_(0C) is determined. Then, the fluoropolymer electrolyte isformed into a film by a technique such as casting film formation, andthe film is similarly subjected to Fourier transform infraredspectroscopy. Thereby, the intensity (I_(1C)) at an absorption peakderived from CF₂ and the intensity (I_(1S)) at an absorption peakderived from SO₂F groups are measured and the ratio A₁=I_(1S)/I_(1C) isdetermined. The ratio (the number of SO₂F groups)/(the number of SO₃Zgroups) in the fluoropolymer electrolyte can be calculated as A₁/A₀.

In general, the number of SO₂F groups in the fluoropolymer electrolyteis as small as negligible in comparison with the number of SO₃Z groups.Thus, the number of SO₃Z groups in the polymer electrolyte can beregarded as the number of SO₂F groups in the precursor ((the number ofSO₃Z groups in polymer electrolyte)=(the number of SO₂F groups inprecursor)).

The fluoropolymer electrolyte preferably has 10 to 95 mol % of the SO₃Zgroup-containing monomer units in the whole monomer units. The term“whole monomer units” herein means all the portions derived from themonomers in terms of the molecular structure of the fluoropolymerelectrolyte.

The SO₃Z group-containing monomer unit is generally derived from an SO₃Zgroup-containing monomer represented by the following formula (I):

CF₂═CF(CF₂)_(k)—O₁—(CF₂CFY¹—O)_(n)—(CFY²)_(m)-A¹  (I)

wherein Y¹ is F, Cl, or a perfluoroalkyl group; k is an integer of 0 to2; 1 is 0 or 1; n is an integer of 0 to 8; n Y¹s may be the same as ordifferent from each other; Y² is F or Cl; m is an integer of 0 to 6,provided that if m=0, 1=0 and n=0; m Y^(e)s may be the same as ordifferent from each other; A^(l) is SO₃Z, Z is an alkali metal,alkaline-earth metal, hydrogen, or NR¹R²R³R⁴, wherein R¹, R², R³, and R⁴each are individually a C1-C3 alkyl group or hydrogen.

In formula (I), k is more preferably 0; 1 is more preferably 1; n ismore preferably 0 or 1, and n is further preferably 0; more preferably,Y² is F and m is an integer of 2 to 6; further preferably, Y² is F and mis 2 or 4; particularly preferably, Y² is F and m is 2; Y¹ is preferablyF; and A¹ is preferably SO₃H, for good synthesis and operability.

In the fluoropolymer electrolyte, one type of the SO₃Z group-containingmonomer may be used, or two or more types of the monomers may be used incombination.

The fluoropolymer electrolyte is preferably a copolymer including arepeating unit (α) derived from the SO₃Z group-containing monomer and arepeating unit (β) derived from an ethylenic fluoromonomer that isdifferent from the monomer that provides a repeating unit (α).

The ethylenic fluoromonomer that provides a repeating unit (β) is amonomer which is free from an ether oxygen [—O—] and which has a vinylgroup. In the vinyl group, part or all of the hydrogen atoms may bereplaced with fluorine atoms.

The term “ether oxygen” herein means the structure —O— constituting themonomer molecule.

Examples of the ethylenic fluoromonomer include haloethylenicfluoromonomers represented by the following formula (II):

CF₂═CF—Rf¹  (II)

wherein Rf¹ is F, Cl, or a C1-C9 linear or branched fluoroalkyl group,and hydrogen-containing fluoroethylenic fluoromonomers represented bythe following formula (III):

CHY³═CFY⁴  (III)

wherein Y³ is H or F; and Y⁴ is H, F, Cl, or a C1-C9 linear or branchedfluoroalkyl group.

Examples of the ethylenic fluoromonomer include tetrafluoroethylene[TFE], hexafluoropropylene [HFP], chlorotrifluoroethylene [CTFE], vinylfluoride, vinylidene fluoride [VDF], trifluoroethylene,hexafluoroisobutylene, and perfluorobutylethylene. Preferable are TFE,VDF, CTFE, trifluoroethylene, vinyl fluoride, and HFP. More preferableare TFE, CTFE, and HFP. Further preferable are TFE and HFP. Particularlypreferable is TFE. Each of the ethylenic fluoromonomers may be usedalone, or two or more of these may be used in combination.

The fluoropolymer electrolyte is preferably a copolymer containing 10 to95 mol % of the repeating unit (α) derived from the SO₃Zgroup-containing monomer, 5 to 90 mol % of the repeating unit (β)derived from the ethylenic fluoromonomer, and 95 to 100 mol % in totalof the repeating unit (α) and the repeating unit (β) in the wholemonomer units.

With respect to the amount of the repeating unit (α) derived from theSO₃Z group-containing monomer, the lower limit is more preferably 15 mol%, and further preferably 20 mol %, while the upper limit is morepreferably 60 mol %, and further preferably 50 mol %.

With respect to the amount of the repeating unit (β) derived from theethylenic fluoromonomer, the lower limit is more preferably 35 mol %,further preferably 40 mol %, and particularly preferably 45 mol %. Thelower limit of the amount of the repeating unit (β) is also preferably50 mol %. The upper limit thereof is more preferably 85 mol %, andfurther preferably 80 mol %.

The fluoropolymer electrolyte in the present invention may have, as athird monomer component other than the above monomers, a repeating unit(γ) derived from a vinyl ether which is not the SO₃Z group-containingmonomer in an amount of preferably 0 to 5 mol %, more preferably 4 mol %or less, and further preferably 3 mol % or less.

The polymer composition of the fluoropolymer electrolyte may becalculated from the value measured in high-temperature NMR at 300° C.

The vinyl ether which is not the SO₃Z group-containing monomer and whichprovides a repeating unit (γ) is not particularly limited as long as itis free from an SO₃Z group. Examples thereof include fluorovinyl ethers,more preferably perfluorovinyl ethers, represented by the followingformula (IV):

CF₂═CF—O—Rf²  (IV)

wherein Rf² is a C1-C9 fluoroalkyl group or a C1-C9 fluoropolyethergroup, and hydrogen-containing vinyl ethers represented by the followingformula (V):

CHY⁵═CF—O—Rf³  (V)

wherein Y⁵ is H or F; Rf³ is a C1-C9 linear or branched fluoroalkylgroup which may have an ether group. Each of the vinyl ethers may beused alone, or two or more thereof may be used in combination.

The electrolyte emulsion preferably contains 2 to 80% by mass (in solidsconcentration) of the fluoropolymer electrolyte. If the amount of thefluoropolymer electrolyte is too small, the amount of an aqueous mediumis too large and, in the case of film formation, the productivity may bepoor. If the amount of the fluoropolymer electrolyte is too large, theviscosity is so high that handling of the emulsion is likely to bedifficult. The lower limit of the amount is more preferably 5% by mass,while the upper limit thereof is more preferably 60% by mass.

The aqueous medium may contain only water. If the electrolyte emulsionis desired to have good dispersibility, the aqueous medium may containan organic polar solvent in addition to water. Examples of the organicpolar solvent include alcohols such as methanol, ethanol, n-propanol,and isopropanol; nitrogen-containing solvents such asN-methylpyrrolidone [NMP]; ketones such as acetone; esters such as ethylacetate; polar ethers such as diglyme and tetrahydrofuran [THF]; andcarbonate esters such as diethylene carbonate. Each of these organicpolar solvents may be used alone, or two or more of these may be used incombination. The aqueous medium may contain an alcohol for improving aleveling ability and a polyoxyethylene for improving a film-formingability for the purpose of film formation by casting, impregnation, orthe like as mentioned below.

The aqueous medium preferably has a water content of higher than 50% bymass. Too low a water content is not preferable because it is likely tocause poor dispersibility and have a bad influence on the environmentand human bodies. The lower limit of the water content is morepreferably 60% by mass, and further preferably 70% by mass, and it isalso preferably 100% by mass. The aqueous medium also preferablycontains 70 to 100% by mass of water and 30 to 0% by mass of one or morealcohols.

The following will describe a method for producing the electrolyteemulsion of the present invention.

(Method for Producing Electrolyte Emulsion)

The electrolyte emulsion of the present invention may be produced by thefollowing method. The present invention also relates to a method forproducing an electrolyte emulsion. The method comprises:

Step (1) in which an ethylenic fluoromonomer and a fluorovinyl compoundhaving an SO₂Z¹ group (Z¹ is a halogen element) are copolymerized at apolymerization temperature of 0° C. or higher and 40° C. or lower toprovide a precursor emulsion containing a fluoropolymer electrolyteprecursor; and

Step (2) in which a basic reactive liquid is added to the precursoremulsion and the fluoropolymer electrolyte precursor is chemicallytreated, whereby an electrolyte emulsion with a fluoropolymerelectrolyte dispersed therein is provided. In this method, theelectrolyte emulsion has an equivalent weight (EW) of 250 or more and700 or less.

The present inventors have performed various studies, and have found thefollowing. That is, a low-EW fluoropolymer electrolyte has many protonexchangeable groups and its volume is greatly increased by an organicsolvent, resulting in problems which cause poor productivity; forexample, a large amount of an alkaline aqueous solution is required or ahigh-concentration alkaline aqueous solution is required. In addition, awashing process after hydrolysis is extremely complicated. Theseproblems have not been found until the production of a low-EW polymerelectrolyte.

The method for producing an electrolyte emulsion of the presentinvention is devised after the present inventors have found thataddition of a basic reactive liquid to the precursor emulsion in Step(2) enables to solve the problems, and thus an electrolyte emulsion witha low-EW fluoropolymer electrolyte dispersed therein can be easily andefficiently produced under mild conditions.

Further, the above production method does not include a step ofre-dispersing or dissolving an electrolyte which has been oncecoagulated by a complicated operation such as sufficient stirring underheating. Thus, the method is excellent in productivity.

The fluoropolymer electrolyte precursor in the precursor emulsionobtained in Step (1) is preferably a spherical particulate substancehaving an average particle size of 10 to 500 nm.

It is important that the above production method does not include anoperation of coagulating particles in the emulsion, such as flocculationor drying. If particles coagulate even only once, it is impossible tofinally provide an electrolyte emulsion with a spherical particulatesubstance having an average particle size of 10 to 500 nm dispersedtherein.

The fluoropolymer electrolyte precursor preferably has a group which isconvertible into SO₃Z (Z is an alkali metal, an alkaline-earth metal,hydrogen, or NR¹R²R³R⁴, and R¹, R², R³, and R⁴ each are individually aC1-C3 alkyl group or hydrogen) by chemical treatment.

In Step (1), the fluoropolymer electrolyte precursor is preferablyprovided by copolymerizing an ethylenic fluoromonomer and a fluorovinylcompound having an SO₂Z¹ group (Z¹ is a halogen element) which isconvertible into SO₃Z (Z is an alkali metal, an alkaline-earth metal,hydrogen, or NR¹R²R³R⁴, and R¹, R², R³, and R⁴ each are individually aC1-C3 alkyl group or hydrogen) by chemical treatment (hereinafter,referred to simply as a fluorovinyl compound).

The fluorovinyl compound is preferably a fluorovinyl compoundrepresented by the following formula (VI):

CF₂═CF(CF₂)_(k)—O₁—(CF₂CFY¹—O)_(n)—(CFY²)_(m)-A²  (VI)

wherein Y¹ is F, Cl or a perfluoroalkyl group; k is an integer of 0 to2; 1 is 0 or 1; n is an integer of 0 to 8; n Y¹ s may be the same as ordifferent from each other; Y² is F or Cl; m is an integer of 0 to 6,provided that if m=0, 1=0 and n=0; m Y^(e)s may be the same as ordifferent from each other; A² is SO₂Z¹, and Z¹ is a halogen element.

In formula (VI), k is preferably 0 and 1 is preferably 1 for goodsynthesis and operability. In order to achieve a low EW, n is morepreferably 0 or 1, and n is further preferably 0. More preferably, Y² isF and m is an integer of 2 to 6. Further preferably, Y² is F and m is 2or 4. Particularly preferably, Y² is F and m is 2. Y¹ is preferably F.

Specific examples of the fluorovinyl compound represented by formula(VI) include CF₂═CFO(CF₂)_(p)—SO₂F, CF₂═CFOCF₂CF(CF₃)O(CF₂)_(p)—SO₂F,CF₂═CF(CF₂)_(P-1)—SO₂F, and CF₂═CF(OCF₂CF(CF₃))_(P)—(CF₂)_(P-1)—SO₂F,wherein P is an integer of 1 to 8.

In Step (1), each of the fluorovinyl compounds may be used alone or twoor more of these may be used in combination.

The ethylenic fluoromonomer may be any of the aforementioned substances.If desired, a third monomer component other than the ethylenicfluoromonomer and the fluorovinyl compound may be polymerized.

In order to finally provide a fluoropolymer electrolyte as a sphericalparticulate substance having an average particle size of 10 to 500 nm,the polymerization method in Step (1) must be a polymerization method inwhich an aqueous solution of a surfactant is used as a polymerizationsolvent, and a fluorovinyl compound and a gaseous ethylenicfluoromonomer are reacted with each other in a state that the componentsare filling-dissolved in the polymerization solvent (that is, emulsionpolymerization). Solution polymerization, bulk polymerization, andsuspension polymerization cannot provide a fluoropolymer electrolyte asa spherical particulate substance having an average particle size of 10to 500 nm. Also in order to efficiently provide a polymer having anequivalent weight (EW) of 250 or more and 700 or less, the emulsionpolymerization is preferable.

The emulsion polymerization may be a polymerization method in which anaqueous solution of a surfactant and a coemulsifier such as an alcoholis used, and a fluorovinyl compound and a gaseous ethylenicfluoromonomer are reacted with each other in a state that the componentsare filling-emulsified in this aqueous solution (mini-emulsionpolymerization or micro-emulsion polymerization). The mini-emulsionpolymerization and the micro-emulsion polymerization lead to a higherapparent polymerization rate.

In Step (1), the ethylenic fluoromonomer and the fluorovinyl compoundare preferably copolymerized at a polymerization temperature of 0° C. orhigher and 40° C. or lower. In addition, Step (1) is preferably a stepwhich provides a fluoropolymer electrolyte precursor emulsion byemulsion polymerization at a polymerization temperature of 0° C. orhigher and 40° C. or lower. Although the reason is not clear, apolymerization reaction at the above polymerization temperature enablesto adjust the distance between ionic clusters of the fluoropolymerelectrolyte in the aforementioned specific range, and thus highconductivity is achieved even at low humidity. The polymerizationtemperature is more preferably 5° C. or higher and 35° C. or lower.

The emulsion polymerization is preferably a polymerization method inwhich, in an aqueous solution of a surfactant prepared in a pressurecontainer, a fluorovinyl compound and a gaseous ethylenic fluoromonomerare radical-copolymerized using radicals generated from a polymerizationinitiator. The fluorovinyl compound may be filling-emulsified by astrong shearing force with a surfactant and a coemulsifier such as analcohol.

In order to control the composition of a polymer to be generated, themethod is preferably one which is capable of controlling a pressurederived from the gaseous ethylenic fluoromonomer. The pressure ispreferably −0.05 MPaG or higher and 2.0 MPaG or lower. The pressure(MPaG) herein is a value on a pressure gauge (gauge pressure) with theatmospheric pressure as 0 MPa. In order to achieve a low EW, thepressure is preferably low in general, but too low a pressure may causea long polymerization time, and thus the process may be inefficient. Thelower limit of the pressure is more preferably 0.0 MPaG, and furtherpreferably 0.1 MPaG. The upper limit thereof is more preferably 1.0MPaG, and further preferably 0.7 MPaG.

Further, in general, the gaseous ethylenic fluoromonomer is consumed andthe pressure decreases, as the polymerization reaction proceeds. Thus,the gaseous ethylenic fluoromonomer is preferably added as appropriate.In addition, the method of additionally supplying the fluorovinylcompound simultaneously consumed is also preferably used. Thefluorovinyl compound to be added may be filling-emulsified by a strongshear force with a surfactant and a coemulsifier such as an alcohol. Ifthe fluorovinyl compound is a liquid, it may be injected using ametering pump or under pressure by an inert gas, for example.

The fluoropolymer electrolyte precursor is preferably melt-flowable. Inthe present embodiments, the melt-flow rate (hereinafter abbreviated as“MFR”) may be used as an indicator of the polymerization degree of thefluoropolymer electrolyte precursor. In the present embodiments, the MFRof the fluoropolymer electrolyte precursor is preferably 0.01 (g/10 min)or higher, more preferably 0.1 (g/10 min) or higher, and furtherpreferably 0.3 (g/10 min) or higher. The upper limit of the MFR ispreferably 100 (g/10 min) or lower, more preferably 20 (g/10 min) orlower, further preferably 16 (g/10 min) or lower, and particularlypreferably 10 (g/10 min) or lower. An MFR of lower than 0.01 (g/10 min)may cause failure in molding processes such as film formation. An MFR ofhigher than 100 (g/10 min) may cause low strength of a film obtained bymolding the precursor, and may cause poor durability if the precursor isused for fuel cells.

In order to adjust the MFR to 0.01 (g/10 min) or higher and 100 (g/10min) or lower, the emulsion polymerization is preferably performed at atemperature of 0° C. or higher and 40° C. or lower. If the temperatureis higher than 40° C., disproportionation, in which radicals of polymerends are β-transformed and the polymerization stops, proceeds at ahigher rate, so that a high-molecular-weight polymer may not beprovided. The temperature is more preferably 35° C. or lower, andfurther preferably 30° C. or lower. On the other hand, if thetemperature is lower than 0° C., the polymerization may proceed veryslowly and the productivity may be very poor. The temperature is morepreferably 5° C. or higher, and further preferably 10° C. or higher.

The polymerization initiator used in Step (1) is preferably awater-soluble one. Examples thereof include inorganic peroxides such aspersulfuric acid compounds, perboric acid compounds, perchloric acidcompounds, perphosphoric acid compounds, and percarbonic acid compounds;and organic peroxides such as disuccinyl peroxides, t-butyl permaleate,and t-butyl hydroperoxide. Examples of the inorganic peroxides mayinclude ammonium salts, sodium salts, and potassium salts.

Combination of any of the water-soluble polymerization initiators and areducing agent, that is, a redox initiator, may be suitably used.Examples of the reducing agent include sulfites, bisulfites, salts oflow-valence ions such as iron, copper, and cobalt, hypophosphorous acid,hypophosphites, organic amines such asN,N,N′,N′-tetramethylethylenediamine, and reducing sugars such asaldoses and ketoses. Particularly in the case that the polymerizationtemperature is 30° C. or lower, a redox initiator is preferably used.

Azo compounds are also most preferable initiators in the presentinvention. Examples thereof include 2,2′-azobis-2-methylpropionamidinehydrochloride, 2,2′-azobis-2, 4-dimethylvaleronitrile,2,2′-azobis-N,N′-dimethyleneisobutylamidine hydrochloride,2,2′-azobis-2-methyl-N-(2-hydroxyethyl)-propionamide,2,2′-azobis-2-(2-imidazolin-2-yl)-propane and salts thereof,4,4′-azobis-4-cyanovaleric acid and salts thereof. Further, two or moreof the aforementioned polymerization initiators may be used incombination. The amount of the polymerization initiator is about 0.001to 5% by mass to the monomer.

The polymerization initiator may be put into a pressure container beforethe introduction of the ethylenic fluoromonomer, or may be injected inan aqueous solution form after the introduction thereof.

In the case of a redox initiator, a polymerization initiator and/or areducing agent are/is preferably added in succession.

The emulsifier used in Step (1) is not particularly limited and ispreferably one having a less chain-transferring ability. For example, anemulsifier represented by RfZ³ may be used. Here, Rf is a C4-C20 alkylgroup, part or all of hydrogen atoms therein are replaced with fluorine,it may have an ether oxygen, and may have an unsaturated bondcopolymerizable with the ethylenic fluoromonomer. Z³ is a dissociativepolar group, and —COO⁻B⁺ or —SO₃ ⁻B⁺ is preferably used. Here, B⁺ is amonovalent cation such as an alkali metal ion, ammonium ion, or hydrogenion.

Examples of the emulsifier represented by RfZ³ include Y(CF₂)_(n)COO⁻B⁺(n is an integer of 4 to 20, Y is fluorine or hydrogen),CF₃—OCF₂CF₂—OCF₂CF₂COO⁻B⁺, and CF₃—(OCF(CF₃)CF₂)_(n)COO⁻B⁺ (n is aninteger of 1 to 3).

The amount of the emulsifier is not particularly limited. It ispreferably 0.01% by mass or more and 10% by mass or less in an aqueoussolution. The larger the amount of the emulsifier is, the more thepolymerized particles tend to be, and the higher the apparentpolymerization rate tends to be. If the amount is less than 0.01% bymass, the emulsified particles may not be stably maintained. If theamount is more than 10% by mass, washing in the post-process isdifficult. The lower limit of the amount is more preferably 0.05% bymass, and further preferably 0.1% by mass. The upper limit thereof ismore preferably 5% by mass, and further preferably 3% by mass.

In Step (1), what is called “seed polymerization”, whereinpolymerization is performed using a large amount of an emulsifier toprovide a dispersion, and the obtained dispersion is diluted andpolymerization is continued, may be performed in order to increase thenumber of polymerized particles.

The polymerization duration is not particularly limited, and isgenerally 1 to 48 hours. The polymerization pH is also not particularlylimited, and may be adjusted during the polymerization, if necessary.Examples of a pH adjuster usable in this case include alkalizing agentssuch as sodium hydroxide, potassium hydroxide, and ammonia, mineralacids such as phosphoric acid, sulfuric acid, and hydrochloric acid, andorganic acids such as formic acid and acetic acid.

In addition, a chain transfer agent may be used so as to adjust themolecular weight and molecular weight distribution. Preferable examplesof the chain transfer agent include gaseous hydrocarbons such as ethaneand pentane, water-soluble compounds such as methanol, and iodinecompounds. In particular, iodine compounds are suitable because theyenable to produce a block polymer by what is called iodine transferpolymerization.

Since a greater molecular weight of the fluoropolymer electrolyte leadsto higher durability, no chain transfer agent is preferably used in Step(1).

The precursor emulsion preferably contains 2 to 80% by mass (in solidsconcentration) of the fluoropolymer electrolyte precursor. The lowerlimit of the amount is more preferably 5% by mass, and the upper limitthereof is more preferably 60% by mass.

In order to improve the durability of the fluoropolymer electrolyteprovided by the production method of the present invention, unstableterminal groups of the fluoropolymer electrolyte precursor in theprecursor emulsion provided in Step (1) may be stabilized. The unstableterminal groups of the fluoropolymer electrolyte precursor may be, forexample, carboxylic acids, carboxylic acid salts, carboxylic acidesters, carbonates, hydrocarbons, and methylol, and may depend on typesand the like of the polymerization method, initiator, chain transferagent, and polymerization terminator to be used.

In the case that emulsion polymerization is selected as thepolymerization method and no chain transfer agent is used, most of theunstable terminal groups are carboxylic acids.

The method of stabilizing the unstable terminal groups of thefluoropolymer electrolyte precursor is not particularly limited. Forexample, the groups are heat-decarboxylated and stabilized as —CF₂Hgroups.

The method for producing an electrolyte emulsion of the presentinvention comprises Step (2) in which a basic reactive liquid is addedto the precursor emulsion provided in Step (1) and the fluoropolymerelectrolyte precursor is chemically treated, whereby an electrolyteemulsion with the fluoropolymer electrolyte dispersed therein isprovided.

Step (2) is a step in which a basic reactive liquid is added to theprecursor emulsion, and thereby the fluoropolymer electrolyte precursorand the basic reactive liquid are made in contact with each other, sothat the fluoropolymer electrolyte precursor is chemically treated toprovide an electrolyte emulsion. Examples of the chemical treatmentinclude hydrolysis and acid treatment. Hydrolysis may be performed byadding a basic reactive liquid to the precursor emulsion.

The basic reactive liquid is not particularly limited, and is preferablyan aqueous solution of a hydroxide of an alkali metal or alkaline-earthmetal such as sodium hydroxide or potassium hydroxide. The amount of thehydroxide of an alkali metal or alkaline-earth metal is not particularlylimited, and is preferably 10 to 30% by mass. The reaction liquidpreferably contains a swellable organic compound such as methyl alcohol,ethyl alcohol, acetone, DMSO, DMAC, and DMF. The amount of the swellableorganic compound is preferably 1 to 50% by mass. The treatmenttemperature depends on the type of a solvent, solvent composition, andthe like conditions; the higher the temperature is, the shorter thetreatment duration is. If the treatment temperature is too high, thepolymer electrolyte precursor may be dissolved and is difficult tohandle in such a case. Thus, the temperature is preferably 20° C. to160° C. Further, in order to achieve high conductivity, all offunctional groups convertible into SO₃H are preferably hydrolyzed.Therefore, the treatment duration is preferably as long as possible. Toolong a duration, however, may cause poor productivity, and thus theduration is preferably 0.5 to 48 hours.

In Step (2), it is also preferable to provide a protonated fluoropolymerelectrolyte by sufficiently washing the product after the hydrolysiswith hot water, if necessary, and then acid-treating the product. Anacid used in the acid treatment is not particularly limited as long asit is selected from mineral acids such as hydrochloric acid, sulfuricacid, and nitric acid, and organic acids such as oxalic acid, aceticacid, formic acid, and trifluoroacetic acid.

In Step (2), the acid treatment is also preferably a treatment in whichthe precursor emulsion or the hydrolyzed precursor emulsion is made incontact with a cation exchange resin. For example, the acid treatmentmay be performed by passing the precursor emulsion or the hydrolyzedprecursor emulsion through a container filled with a cation exchangeresin.

The method for producing an electrolyte membrane is also one aspect ofthe present invention, the method comprising the steps of applying theelectrolyte emulsion to a substrate, drying the electrolyte emulsionapplied to the substrate to provide an electrolyte membrane, and peelingthe electrolyte membrane off from the substrate. The above productionmethod can provide an electrolyte membrane having a lower water contentin comparison with the case of producing an electrolyte membrane fromthe electrolyte solution.

The aforementioned method for producing an electrolyte membrane is amethod called casting film formation. Examples thereof include a methodin which an electrolyte emulsion is developed on a container such as alaboratory dish; the emulsion is heated in, for example, an oven ifnecessary, so that the solvent is at least partially distilled off; andthe dried emulsion is separated, for example, from the container, sothat a film-shaped product is obtained. Examples thereof further includea method in which an electrolyte emulsion is cast on a substrate such asa glass plate or a film in a manner such that the thickness is uniformusing a device such as a blade coater, gravure coater, or comma coaterhaving a mechanism such as a blade, air knife, or reverse roll, so thata sheet-formed film is obtained. In addition, examples thereof mayinclude a method of continuous casting film formation, so that a longfilm-shaped membrane is obtained.

The film is not particularly limited. The film may be prepared from amaterial selected from polyesters including polyethylene terephthalate(PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN),and liquid crystal polyesters, triacetyl cellulose (TAC), polyarylate,polyether, polycarbonate (PC), polysulfone, polyethersulfone,cellophane, aromatic polyamide, polyvinyl alcohol, polyethylene (PE),polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS),acrylonitrile-butadiene-styrene copolymer (ABS), polymethyl methacrylate(PMMA), polyamide, polyacetal (POM), polyphenylene terephthalate (PPE),polybutylene terephthalate (PBT), polyphenylene sulfide (PPS),polyamide-imide (PAI), polyether imide (PEI), polyether ether ketone(PEEK), polyimide (PI), polymethyl pentene (PMP),polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene (FEP),tetrafluoroethylene-ethylene (ETFE) copolymer, polyvinylidene fluoride(PVDF), polybenzazole (PBZ), polybenzoxazole (PBO), polybenzothiazole(PBT), polybenzimidazole (PBI), and poly-paraphenylene terephthalamide(PPTA).

The electrolyte membrane may be produced by the aforementioned castingfilm formation.

An electrolyte membrane obtainable by the above production method isalso one aspect of the present invention. The thickness of theelectrolyte membrane is preferably 1 μm or higher and 500 μm or lower,more preferably 2 μm or higher and 100 μm or lower, and furtherpreferably 5 μm or higher and 50 μm or lower. A thin membrane can causea low DC resistance upon power generation, but may also cause a largegas permeation amount. Thus, the thickness is preferably within theabove appropriate range. In addition, the membrane may have a porousfilm prepared by extending a PTFE film as disclosed in JP 08-162132 A orfibrillated fibers disclosed in JP 53-149881 A and JP 63-61337 B.

The electrolyte emulsion of the present invention may be used as anelectrolyte binder in an electrode catalyst layer. An electrode catalystlayer comprising the electrolyte emulsion is also one aspect of thepresent invention. In this case, an electrode catalyst layer ispreferably produced by mixing the fluoropolymer electrolyte emulsion ofthe present invention and an electrode catalyst such as carbon-supportedPt to provide an electrode ink (electrode catalyst composition),applying the electrode ink to a substrate, and drying the electrode ink.The amount of the fluoropolymer electrolyte to be supported with respectto the electrode area is preferably 0.001 to 10 mg/cm², more preferably0.01 to 5 mg/cm², and further preferably 0.1 to 1 mg/cm², in a statethat an electrode catalyst layer is formed.

The electrode catalyst layer comprises a composite particulate substancewhich comprises fine particles of a catalyst metal and a conductiveagent supporting the fine particles, and a polymer electrolyte as abinder. It may contain a water repellent, if necessary. The catalystmetal to be used for an electrode may be any metal which promotesoxidation of hydrogen and reduction of oxygen. The catalyst metal ispreferably at least one metal selected from the group consisting ofplatinum, gold, silver, palladium, iridium, rhodium, ruthenium, iron,cobalt, nickel, chromium, tungsten, manganese, vanadium, and any alloyof these metals. Mainly used among these is platinum.

The conductive agent is not particularly limited as long as it is aparticulate substance having conductivity (conductive particulatesubstance). The conductive particulate substance is preferably at leastone conductive particulate substance selected from the group consistingof carbon blacks such as furnace black, channel black, and acetyleneblack, active carbon, graphite, and various metals. The particle size ofthe conductive agent is preferably 10 angstrom to 10 μm, more preferably50 angstrom to 1 μm, and most preferably 100 to 5,000 angstrom. Theparticle size of the fine particles of a catalyst metal (electrodecatalyst particles) is not particularly limited, and is preferably 10 to1,000 angstrom, more preferably 10 to 500 angstrom, and most preferably15 to 100 angstrom.

In the composite particulate substance, the amount of the electrodecatalyst particles to the conductive particulate substance is preferably1 to 99% by mass, more preferably 10 to 90% by mass, and most preferably30 to 70% by mass. Specifically, suitable examples thereof includecarbon-supported Pt catalyst such as TEC10E40E (Tanaka Kikinzoku KogyoK.K.).

The amount of the composite particulate substance is 20 to 95% by mass,preferably 40 to 90% by mass, more preferably 50 to 85% by mass, andmost preferably 60 to 80% by mass, to the whole mass of the electrodecatalyst layer.

The amount of the electrode catalyst to be supported with respect to theelectrode area is preferably 0.001 to 10 mg/cm², more preferably 0.01 to5 mg/cm², and most preferably 0.1 to 1 mg/cm², in a state that theelectrode catalyst layer is formed. The thickness of the electrodecatalyst layer is preferably 0.01 to 200 μm, more preferably 0.1 to 100μm, and most preferably 1 to 50 μm.

The porosity of the electrode catalyst layer is not particularlylimited, and is preferably 10 to 90% by volume, more preferably 20 to80% by volume, and most preferably 30 to 60% by volume.

For improved repellency, the electrode catalyst layer of the presentinvention may further contain polytetrafluoroethylene (hereinafter,referred to as PTFE). In this case, PTFE may have any finite form, andis preferably in a particulate or fibrous state. One type of PTFE may beused alone, or two or more types of PTFE may be used in combination.

In the case that the electrode catalyst layer contains PTFE, the amountof the PTFE is preferably 0.001 to 20% by mass, more preferably 0.01 to10% by mass, and most preferably 0.1 to 5% by mass, to the whole mass ofthe electrode catalyst layer. For improved hydrophilicity, the electrodecatalyst layer of the present invention may further contain a metaloxide. In this case, the metal oxide is not particularly limited, and ispreferably at least one metal oxide selected from the group consistingof Al₂O₃, B₂O₃, MgO, SiO₂, SnO₂, TiO₂, V₂O₅, WO₃, Y₂O₃, ZrO₂, Zr₂O₃, andZrSiO₄. In particular, the metal oxide is preferably at least one metaloxide selected from the group consisting of Al₂O₃, SiO₂, TiO₂, and ZrO₂.Particularly preferable is SiO₂.

In the case that the electrode catalyst layer contains a metal oxide inthe embodiments of the present invention, the amount of the metal oxideis preferably 0.001 to 20% by mass, more preferably 0.01 to 10% by mass,and most preferably 0.1 to 5% by mass, to the whole mass of theelectrode catalyst layer. The metal oxide may be in a particulate orfibrous state; in particular, it is preferably in an infinite form. Theterm “infinite form” herein means a form that neither particulate norfibrous metal oxide is found under optical microscope or electronmicroscope observation. Even though the electrode catalyst layer isobserved under several hundred thousand times magnification using ascanning electron microscope (SEM), neither particulate nor fibrousmetal oxide is observed. Further, even though the electrode catalystlayer is observed under several hundred thousand to several milliontimes magnification using a transmission electron microscope (TEM),neither particulate nor fibrous metal oxide is clearly observed. Asmentioned here, the term “infinite form” herein means that neitherparticulate nor fibrous metal oxide is observed with the currentmicroscope technology.

Use of the aforementioned electrode catalyst layer enables to suppressflooding and to generate high power. This is presumably because, in thiscase, the water content is low and the electrode is excellent indraining performance.

(Method for Producing Electrode Catalyst Layer)

The following will describe a method for producing an electrode catalystlayer. The present invention also relates to a method for producing anelectrode catalyst layer, the method comprising the steps of: dispersingcomposite particles comprising a catalyst metal and a conductive agentin the electrolyte emulsion to prepare an electrode catalystcomposition; applying the electrode catalyst composition to a substrate;and drying the electrode catalyst composition applied to the substrateto provide an electrode catalyst layer. The present invention furtherrelates to an electrode catalyst layer obtainable by this productionmethod. For example, the electrode catalyst layer may be produced asfollows: an electrolyte emulsion is prepared; the composite particulatesubstance is dispersed into this electrolyte emulsion to prepare anelectrode catalyst composition; this composition is applied to a polymerelectrolyte membrane or to another substrate such as a PTFE sheet; andthe applied composition is dried and solidified. In the presentinvention, the electrode catalyst composition can be applied by anyconventionally known method such as screen printing or spraying. Theelectrode catalyst composition contains a fluoropolymer electrolyte, acomposite particulate substance, and an aqueous medium.

The electrode catalyst composition may further contain a solvent, ifnecessary. Examples of the solvent to be used include single solventssuch as water, alcohols (e.g. ethanol, 2-propanol, ethylene glycol, andglycerin), and chlorofluorocarbons, and composite solvents thereof. Theamount of the solvent is preferably 0.1 to 90% by mass, more preferably1 to 50% by mass, and most preferably 5 to 20% by mass, to the wholemass of the electrode catalyst composition.

On the other hand, the electrode catalyst layer of the present inventionmay also be produced as follows: the electrode catalyst composition isapplied to a gas diffusion electrode formed by stacking a gas diffusionlayer and an electrode catalyst layer, such as ELAT (registeredtrademark, BASF), or the gas diffusion electrode is immersed in theelectrode catalyst composition so that the composition is applied to theelectrode; and the composition is dried and solidified.

In addition, the electrode catalyst layer may be immersed in aninorganic acid such as hydrochloric acid after the production. Theacid-treatment temperature is preferably 5° C. to 90° C., morepreferably 10° C. to 70° C., and most preferably 20° C. to 50° C.

A unit in which two types of electrode catalyst layers, an anode and acathode, are joined to the respective faces of the electrolyte membraneis called a membrane electrode assembly (hereinafter, also referred toas “MEA”). A membrane electrode assembly comprising the electrolytemembrane of the present invention is also one aspect of the presentinvention. Further, a membrane electrode assembly comprising theelectrode catalyst layer of the present invention is also one aspect ofthe present invention. A unit in which a pair of gas diffusion layersare oppositely joined to the outer faces of the electrode catalystlayers is also called an MEA in some cases.

The MEA obtained as mentioned above, in some cases the MEA with a pairof gas diffusion electrodes oppositely joined, is assembled with othercomponents used for common polymer electrolyte fuel cells, such as abipolar plate and a bucking plate, and thereby a polymer electrolytefuel cell is formed.

A bipolar plate is a plate made of a composite material of graphite andresin, or a metal, and grooves are formed on its surface as passages forfuels and gases, such as an oxidant. The bipolar plate has functions asa transmitter of electrons to an outside load circuit and a channel forsupplying fuels and oxidants to the vicinity of the electrode catalyst.A fuel cell is produced by inserting MEAs between such bipolar platesand stacking them.

An electrolyte membrane can be produced as follows: the electrolyteemulsion of the present invention is mixed with an organic solvent toprepare an electrolyte solution with the fluoropolymer electrolytedissolved therein; this electrolyte solution is applied to a substrate;the electrolyte solution applied to the substrate is dried to form anelectrolyte membrane; and the electrolyte membrane is peeled off fromthe substrate to provide an electrolyte membrane. Further, an electrodecatalyst layer can be produced as follows: the electrolyte emulsion ofthe present invention is mixed with an organic solvent to prepare anelectrolyte solution with the fluoropolymer electrolyte dispersedtherein; a composite particulate substance containing a catalyst metaland a conductive agent is dispersed into this electrolyte solution toprepare an electrode catalyst composition; the electrode catalystcomposition is applied to a substrate; and the electrode catalystcomposition applied to the substrate is dried to form an electrodecatalyst layer.

In the case of first preparing an electrolyte solution and thenproducing an electrolyte membrane or an electrode catalyst layer, amethod called casting film formation can be used. Examples thereofinclude a method in which a polymer electrolyte solution is developed ona container such as a laboratory dish; the emulsion is, if necessary,heated in an oven, for example, so that the solvent is at leastpartially distilled off; and the dried emulsion is, for example,separated from the container, so that a film-shaped product is obtained.Examples thereof further include a method in which a polymer electrolytesolution is cast on a substrate such as a glass plate or a film in amanner such that the thickness is uniform using a device such as a bladecoater, gravure coater, or comma coater having a mechanism such as ablade, air knife, or reverse roll, so that a sheet-formed film isobtained. In addition, examples thereof may include a method in which afilm is continuously cast-formed, so that a long film-shaped membrane isobtained.

The film is not particularly limited. The film may be prepared from amaterial selected from polyesters including polyethylene terephthalate(PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN)and liquid crystal polyesters, triacetyl cellulose (TAC), polyarylate,polyether, polycarbonate (PC), polysulfone, polyethersulfone,cellophane, aromatic polyamide, polyvinyl alcohol, polyethylene (PE),polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS),acrylonitrile-butadiene-styrene copolymer (ABS), polymethyl methacrylate(PMMA), polyamide, polyacetal (POM), polyphenylene terephthalate (PPE),polybutylene terephthalate (PBT), polyphenylene sulfide (PPS),polyamide-imide (PAI), polyether imide (PEI), polyether ether ketone(PEEK), polyimide (PI), polymethyl pentene (PMP),polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene (FEP),tetrafluoroethylene-ethylene (ETFE) copolymer, polyvinylidene fluoride(PVDF), polybenzazole (PBZ), polybenzoxazole (PBO), polybenzothiazole(PBT), polybenzimidazole (PBI), and poly-paraphenylene terephthalamide(PPTA).

Examples of the organic solvent include protonic organic solvents suchas methanol, ethanol, n-propanol, isopropyl alcohol, butanol, andglycerin, and non-protonic solvents such as N,N-dimethyl formamide,N,N-dimethyl acetamide, and N-methyl pyrrolidone. Each of these may beused alone, or two or more of these may be used in combination.

The dissolution method is not particularly limited. For example, a mixedsolvent of water and a protonic organic solvent is first added to anelectrolyte emulsion under the conditions such that the total solidsconcentration is 1 to 50% by mass. Next, this composition is chargedinto an autoclave which may optionally have an inner cylinder made ofglass. The air inside the autoclave is replaced with an inert gas suchas nitrogen, and then the composition is heated and stirred at aninternal temperature of 50° C. to 250° C. for 1 to 12 hours. Thereby, anelectrolyte solution is provided. The total solids concentration ispreferably as high as possible in terms of yield, but too high aconcentration may cause undissolved material. Thus, the concentration ispreferably 1 to 50% by mass, more preferably 3 to 40% by mass, andfurther preferably 5 to 30% by mass.

If a protonic organic solvent is used, the ratio of the protonic organicsolvent to water in the obtained electrolyte solution may beappropriately adjusted depending on the dissolution method, dissolutionconditions, type of a polymer electrolyte, total solids concentration,dissolution temperature, stirring rate, and the like. The amount of theprotonic organic solvent is preferably 10 to 1,000 parts by mass for 100parts by mass of water, and the ratio of the organic solvent isparticularly preferably 10 to 500 parts by mass for 100 parts by mass ofwater.

The electrolyte solution contains one or more types of emulsions (liquidparticles are dispersed in a liquid as colloidal particles or coarserparticles to give a milk-like appearance), suspensions (solid particlesare dispersed in a liquid as colloidal particles or particles which canbe observed using a microscope), colloidal liquids (macromolecules aredispersed), and micellar liquids (a large amount of small moleculesassociate with each other by intermolecular force to form a lyophiliccolloidal dispersion system).

The electrolyte solution may be concentrated. The concentration methodis not particularly limited. For example, the solution may be heated sothat the solvent is distilled off, or the solution may be concentratedunder reduced pressure. With respect to the solids content in theresulting coating solution, too high a content may cause high viscosityand poor handleability, while too low a content may cause lowproductivity. Thus, the final solids content in the coating solution ispreferably 0.5 to 50% by mass.

More preferably, the electrolyte solution is filtered in order to removea coarser particle fraction. The filtration method is not particularlylimited, and any conventional general method can be applied. Typicalexamples thereof include a method in which a filter material showing astandard filter rating for ordinary use is processed into a filter, andthe solution is filtered under pressure using this filter. With respectto the filter, the filter material preferably has a 90% collectionparticle size of 10 to 100 times as large as the average particle sizeof the particles. This filter material may be a filter paper, or may bea sintered metallic filter. Particularly in the case of the filterpaper, the 90% collection particle size is preferably 10 to 50 times aslarge as the average particle size of the particles. In the case of thesintered metallic filter, the 90% collection particle size is preferably50 to 100 times as large as the average particle size of the particles.A 90% collection particle size set to 10 or more times as large as theaverage particle size enables to suppress an excessive increase in thepressure required for liquid delivery and to suppress clogging of thefilter within a short time. On the other hand, a 90% collection particlesize is preferably set to 100 or less times as large as the averageparticle size so as to well remove aggregated particles and undissolvedresin, which may cause contamination in a film.

EXAMPLES

The following will describe the present embodiments in detail referringto examples. The present embodiments are not limited to these examples.

The evaluation methods and measurement methods used in the presentembodiments are as follows.

(EW Measurement)

About 2 to 20 cm² of the polymer electrolyte membrane in which counterions of the ion-exchange group were protons was immersed in a saturatedNaCl aqueous solution (30 ml) at 25° C., and the mixture was left for 30minutes under stirring. Then, the protons in the saturated NaCl aqueoussolution were neutralization-titrated with a 0.01 N sodium hydroxideaqueous solution using phenolphthalein as an indicator. The polymerelectrolyte membrane obtained after the neutralization in which thecounter ions of the ion-exchange group were sodium ions was washed withpure water, vacuum-dried, and weighed. Assuming that the amount ofsubstance of the sodium hydroxide used for neutralization was M (mmol),and that the weight of the polymer electrolyte membrane in which thecounter ions of the ion-exchange group were sodium ions was representedas W (mg), the equivalent weight (EW) (g/eq) was determined by thefollowing formula (2).

EW=(W/M)−22  (2)

(Calculation of Cluster Distance)

The polymer electrolyte membranes were stacked so that the totalthickness was about 0.25 mm, and the laminated sample was mounted on ahumidity-controllable cell for small-angle X-ray scattering. The samplewas maintained under the conditions of 25° C. and 50% RH for 30 minutes.X-rays were applied thereto and the scattering state was measured. Themeasurement conditions were as follows: X-ray wavelength λ: 0.154 nm,camera length: 515 mm; and detector: imaging plate. The two-dimensionalscattering pattern obtained by the imaging plate was adjusted byempty-cell correction and background correction based on the detector,and then subjected to circular averaging. Thereby, a one-dimensionalscattering profile was obtained. In the scattering profile whereinscattering intensities are plotted with respect to Bragg angles θ, theBragg angle θm at the peak position derived from the cluster structurewithin the range of 2θ>1° was read and the distance between ionicclusters was calculated by the following formula (1).

d=λ/2/sin(θm)  (1)

(Measurement of Proton Conductivity)

The proton conductivity was measured as follows using an MSB-AD-V-FCpolymer membrane water content test apparatus (BEL Japan Inc.). Apolymer electrolyte membrane formed with a thickness of 50 μm was cutout into a size of 1 cm in width and 3 cm in length, and this sample wasmounted on a cell for conductivity measurement. Next, the cell forconductivity measurement was placed in a chamber of the tester, and theconditions in the chamber were adjusted to 110° C. and lower than 1% RH.Then, water vapor generated from ion exchange water was introduced intothe chamber so that the inside of the chamber was humidified to 10% RH,30% RH, 50% RH, 70% RH, 90% RH, and 95% RH, in this order. Theconductivities at the respective humidities were measured.

The humidity H at 0.10 S/cm was calculated based on the followingformula β):

H=(H2−H1)/(σ2−σ1)×(0.1−σ1)+H1  (3)

provided that H2 and σ2 are the relative humidity and the conductivity,respectively, at the measurement point first after the conductivityexcesses 0.10 S/cm, and that H1 and σ1 are the highest relative humidityat the measurement point before the conductivity excesses 0.10 S/cm andthe conductivity at that point.

(Method for Measuring Melt-Flow Rate [MFR])

The MFR of the fluoropolymer was measured under the conditions of 270°C. and a load of 2.16 kg in accordance with JIS K 7210 using MELTINDEXER TYPE C-5059D (Toyo Seiki Seisaku-sho, Ltd.). The amount of thepolymer pushed out was represented as grams for 10 minutes.

(Polymer Composition)

The polymer composition was calculated based on the measured value byhigh-temperature NMR at 300° C. The NMR was performed using a Fouriertransform nuclear magnetic resonance (FT-NMR) device AC 300P (BrukerCorporation). The polymer composition was calculated using the peakintensity at around −120 ppm derived from tetrafluoroethylene and vinylether and the peak intensity at around −80 ppm derived from vinyl etherand based on the respective peak integral values.

(Measurement of Average Particle Size and Aspect Ratio)

The average particle size and the aspect ratio were determined asfollows: the electrolyte emulsion was applied to an aluminum foil or thelike substrate, and then the aqueous medium was removed so that anaggregate of the fluoropolymer electrolyte was obtained; this aggregatewas observed using a scanning electron microscope or the like, andthereby an image was obtained; on the obtained image, 20 or moreparticles each were measured for the lengths of its major and minoraxes; the average value of the length ratios (major axis/minor axis) wastreated as the aspect ratio, and the average value of the lengths of themajor and minor axes was treated as the average particle size.

(Method for Measuring Solids Concentration)

A dried weighing bottle at room temperature was precisely weighed, andthe obtained weight was called W0. A 10-g portion of a material to bemeasured was put into the weighed bottle and the total weight wasprecisely weighed. This weight was called W1. The weighing bottle withthe material put therein was dried for 3 hours or longer at atemperature of 110° C. and an absolute pressure of 0.01 MPa or lowerusing an LV-120 type vacuum drier (ESPEC Corp.), and then cooled down ina silica-gel-charged desiccator. After the temperature reached roomtemperature, the weight was precisely measured, and this weight wascalled W2. The ratio (W2−W0)/(W1−W0) was represented in percentage. Themeasurement was performed 5 times, and the average value thereof wastreated as the solids concentration.

Measurement of ratio (the number of SO₂F groups)/(the number of SO₃Zgroups)

The IR measurement was performed on a film formed by heat-pressing theprecursor and a cast membrane formed from the obtained electrolyteemulsion, and thereby the ratio was measured.

(Method for Measuring 25° C. Water Content)

A polymer electrolyte membrane formed with a thickness of about 50 μmwas stored for 1 hour in a constant-temperature and constant-humidityroom controlled at 23° C. and 50% RH, and then cut out into a size of 3cm in length and 4 cm in width. Next, a SUS304-made container filledwith ion exchange water was immersed in a water bath THERMAL ROBO TR-2A(AS ONE Corporation) similarly filled with ion exchange water, so thatthe temperature of the ion exchange water in the SUS304-made containerwas 25° C. After the ion exchange water reached 25° C., theaforementioned polymer electrolyte membrane was immersed in the water.At this time, a polytetrafluoroethylene mesh or the like may also beimmersed therein on the polymer electrolyte membrane in order to preventfloating up of the polymer electrolyte membrane. After 1-hour immersion,the polymer electrolyte membrane was taken out from the water and thewater on the surface was wiped using a filter paper (CatNo. 1441 125,Whatman Ltd.). Then, the weight MW of the polymer electrolyte membranecontaining water was measured to the order of 0.0001 g using anelectronic balance GR-202 (A&D Company, Limited). At this time, in orderto prevent excessive drying of the polymer electrolyte membrane, theweight was measured within 10 seconds after the membrane was taken outfrom the water. Thereafter, the polymer electrolyte membrane was driedat 160° C. for 1 hour using a hot-air drier SPH-101 (ESPEC Corporation),and the weight MD of the dried polymer electrolyte membrane was measuredusing the electronic balance. The ratio (MW-MD)/MD was represented inpercentage, and this value was treated as the 25° C. water content.

(Fuel Cell Evaluation)

In order to examine the cell characteristics (hereinafter referred to as“initial characteristics”) of the electrode catalyst layer and themembrane electrode assembly (MEA) produced as mentioned below, thefollowing fuel cell evaluation was performed.

First, an anode-side gas diffusion layer and a cathode-side gasdiffusion layer were placed opposite to each other and the MEA producedas follows was placed therebetween, and thereby the MEA was assembledinto an evaluation cell. Carbon cloth (DE NORA NORTH AMERICA (USA), ELAT(registered trademark) B-1) was used as the gas diffusion layer on eachof the anode and cathode sides. Next, this evaluation cell was mountedon an evaluation device (CHINO Corporation) and heated to 80° C., andthen hydrogen gas was flowed to the anode side at a rate of 300 cc/minand air gas to the cathode side at 800 cc/min. These gases werepreviously humidified. That is, the hydrogen gas and the air gas werehumidified at a desired temperature by water-bubbling, and then suppliedto the evaluation cell. The evaluation cell was maintained at a voltageof 0.6 V for 20 hours under the conditions of a cell temperature of 80°C. and a desired humidity, and the current was measured.

Example 1 (1.1) Polymerization Step

A fluoroelectrolyte emulsion having repeating units derived from CF₂═CF₂and CF₂═CF—O—(CF₂)₂—SO₃H with an EW of 455 was prepared as follows.

A 6-L-capacity SUS316-made pressure-resistant container provided with astirring wing and a temperature-controlling jacket was charged withwater purified by reverse osmosis membrane (2,850 g), C₇F₁₅COONH₄ (150g), and CF₂═CFOCF₂CF₂SO₂F (1,150 g). The air inside the system wasreplaced with nitrogen, and then the container was evacuated.Subsequently, TFE was introduced therein until the internal pressurereached 0.07 MPaG. The mixture was stirred at 400 rpm, and thetemperature was controlled so that the internal temperature was 10° C. Asolution of (NH₄)₂S₂O₈ (6 g) in water (20 g) was injected therein, and asolution of Na₂SO₃ (0.6 g) in water (20 g) was further injected therein,so that polymerization was initiated. TFE was additionally put so as tomaintain the internal pressure at 0.07 MPaG, and the polymerization wascontinued. Further, a solution of Na₂SO₃ (0.6 g) in water (20 g) wasinjected therein every 1 hour.

After 11 hours from the polymerization initiation, that is, at the timewhen 400 g in total of TFE was additionally put, TFE was released fromthe pressure and the polymerization was stopped. Thereby, 4,700 g of apolymerization liquid (precursor emulsion) was obtained. The obtainedprecursor emulsion had a solids concentration of 24.0% by mass.

Water (250 g) was added to a 200-g portion of the obtainedpolymerization liquid, and the mixture was further mixed with nitricacid to be coagulated. The coagulated polymer was filtered, andre-dispersion in water and filtration was repeated 3 times. Then, thepolymer was dried at 90° C. for 24 hours and subsequently at 120° C. for5 hours using a hot-air drier. Thereby, 44.3 g of a polymer(fluoroelectrolyte precursor) was obtained. The obtained polymer has anMFR of 0.4 g/10 min.

(1.2) Hydrolysis Step

A 2-kg portion of the polymerization liquid (precursor emulsion)obtained in the step (1.1) was diluted 2-fold with pure water. Theliquid was put into a 10-L-capacity three-neck flask and stirredtherein. The temperature was adjusted to 80° C., and the pH was kept at10 or higher while 10% by mass of a sodium hydroxide aqueous solutionwas dropwise added. Thereby, —SO₂F of the fluoropolymer was hydrolyzed.Although reduction in the pH was no longer observed after 3-hourhydrolysis, the hydrolysis was continued for another 2 hours, and thenstopped. No deposition of the fluoropolymer was visually observed duringthe hydrolysis.

(1.3.1) Ultrafiltration Step

Dilute sulfuric acid was added to the reaction solution obtained in thestep (1.2) so that the pH was adjusted to 8, and the solution wasultrafiltered using an ultrafiltration device (Millipore Corporation).The ultrafiltration membrane was one having a molecular weight cutoff of10,000 (Millipore Corporation, Pelicon 2 Filter). This membrane wasinserted in a stainless-steel holder (Millipore Corporation), and thusan ultrafiltration unit was prepared. The reaction solution obtained inthe step (1.2) was put into a 10-L beaker, and was supplied to theultrafiltration unit through a liquid delivery pump (MilliporeCorporation, easy-load Master Flex 1/P). A filtrate containing foreignmatter was discharged from the system, and the treated liquid wasreturned to the beaker. Purified water in an amount equivalent to thatof the removed filtrate was appropriately added to the beaker andultrafiltration was performed again. The addition of pure water wasstopped when the electric conductivity of the filtrate reached 10μS·cm⁻¹, and the ultrafiltration was stopped when the amount of thetreated liquid reached 1 L. Thereby, an aqueous dispersion A wasobtained. The electric conductivity was measured using an electricconductivity meter Twin Cond B-173 (HORIBA, Ltd.). The ultrafiltrationtreatment was continued for 5 hours.

(1.3.2) Ion Exchange

Amberlite IR120B (200 g) (Rohm & Haas Company) was converted into anacid type using sulfuric acid, sufficiently washed with pure water, andcharged into a glass burette. The aqueous dispersion obtained in thestep (1.3.1) (200 g) was passed through the burette over 1 hour, andthereby an acid-type aqueous dispersion B (electrolyte emulsion) wasobtained. The obtained electrolyte emulsion had a solids concentrationof 12.5% by mass, and a viscosity of 20.8 mPa·s measured at 25° C. and ashear rate of 20.4 s⁻¹ using a B-type viscometer. The ratio (the numberof SO₂F groups)/(the number of SO₃Z groups) was 0. The fluoropolymerelectrolyte had an average particle size of 42 nm and an aspect ratio of1.0.

(1.4) Film Formation

The acid-type aqueous dispersion B obtained in the step (1.3.2) wasdeveloped on a glass laboratory dish. The dispersion was heat-dried at80° C. for 30 minutes using NEO HOT PLATE HI-1000 (AS ONE Corporation),and thereby the solvent was removed. The dispersion was furtherheat-treated at 160° C. for 1 hour. Thereafter, the heated dispersionwas immersed in 25° C. ion exchange water and separated from the glasslaboratory dish. Thereby, a fluoropolymer electrolyte membrane having athickness of about 50 μm was obtained. No crease was observed on theobtained electrolyte membrane.

This fluoropolymer electrolyte membrane had an EW of 455.

(1.5) Ion Conductivity, Cluster Distance, and 25° C. Water Content

The fluoropolymer electrolyte membrane obtained in the step (1.4) had adistance between ionic clusters of 2.3 nm and ion conductivities of 0.10S/cm at 110° C. and 25% RH and 0.20 S/cm at 110° C. and 50% RH. Further,the 25° C. water content was 160%; this water content was lower thanthat of the electrolyte membrane in Comparative Example 1 which wasproduced from the fluoropolymer electrolyte solution.

(1.6) Production of Electrode Catalyst Layer 1

An electrode catalyst layer was prepared as follows: an electrolytesolution (0.825 g) (trade name: SS 700C/20, Asahi Kasei E-materialsCorp., polymer weight ratio: 20.0 wt %, solvent: water) comprising afluoropolymer electrolyte having an EW of 720 and ethanol (8.175 g) wereadded to a platinum-supporting catalyst TEC10E40E (0.4 g) (TanakaKikinzoku Kogyo K.K., platinum content: 40 wt %), and they were mixedand stirred so that the mixture was formed into an ink state; theink-state mixture was applied to a PTFE sheet by screen printing; andthe applied mixture was dried and solidified under air atmosphere at160° C. for 1 hour. The platinum content of this electrode catalystlayer was 0.17 mg/cm² in an anode electrode and 0.32 mg/cm² in a cathodeelectrode.

(1.7) Production of Electrode Catalyst Layer 2

An electrode catalyst layer was prepared as follows: the acid-typeaqueous dispersion B obtained in the step (1.3.2) was diluted with ionexchange water and ethanol to prepare a liquid (polymer weight ratio:5.5 wt %, solvent composition (mass ratio): ethanol/water=50/50); a3.0-g portion of this liquid and ethanol (6.0 g) were added to aplatinum-supporting catalyst TEC10E40E (0.4 g) (Tanaka Kikinzoku KogyoK.K., platinum content: 40 wt %), and they were mixed and stirred sothat the mixture was formed into an ink state; the ink-state mixture wasapplied to a PTFE sheet by screen printing; and the applied mixture wasdried and solidified under air atmosphere at 160° C. for 1 hour. Theplatinum content of this electrode catalyst layer was 0.17 mg/cm² in ananode electrode and 0.32 mg/cm² in a cathode electrode.

(1.8) Production of Membrane Electrode Assembly (MEA 1)

The anode electrode and cathode electrode both produced in the step(1.6) were placed opposite to each other, and the fluoropolymerelectrolyte membrane produced in the step (1.4) was placed therebetween.They were hot-pressed under the conditions of 180° C. and a surfacepressure of 0.1 MPa, so that the anode electrode and the cathodeelectrode were printed and joined to the polymer electrolyte membrane.Thereby, an MEA 1 was produced.

(1.9) Production of Membrane Electrode Assembly (MEA 2)

The anode electrode and cathode electrode both produced in the step(1.7) were placed opposite to each other, and a polymer electrolytemembrane (trade name: Aciplex SF7202, Asahi Kasei E-materials Corp.) wasplaced therebetween. They were hot-pressed under the conditions of 180°C. and a surface pressure of 0.1 MPa, so that the anode electrode andthe cathode electrode were printed and joined to the polymer electrolytemembrane. Thereby, an MEA 2 was produced.

(1.10) Fuel Cell Evaluation (MEA 1)

The fuel cell evaluation was performed as mentioned above on the MEA 1produced in the step (1.8). As a result, the current density was as highas 0.57 A/cm² after the MEA was maintained at a voltage of 0.6 V for 20hours under the conditions of a cell temperature of 80° C. and the 50°C. saturated vapor pressure (corresponding to humidity 26% RH).

(1.11) Fuel Cell Evaluation (MEA 2)

The fuel cell evaluation was performed as mentioned above on the MEA 2produced in the step (1.9). As a result, the current density was as highas 0.49 A/cm² after the MEA was maintained at a voltage of 0.6 V for 20hours under the conditions of a cell temperature of 80° C. and the 50°C. saturated vapor pressure (corresponding to humidity 26% RH).

Comparative Example 1

The fluoroelectrolyte precursor obtained in the step (1.1) of Example 1was brought into contact with an aqueous solution of potassium hydroxide(15% by mass) and methyl alcohol (50% by mass) at 80° C. for 20 hours,and was thereby hydrolyzed. In the present step, the fluoropolymerelectrolyte absorbed the aqueous solution in an amount about 13.7 timesas large as the dried weight of the fluoroelectrolyte precursor.Therefore, its volume remarkably increased and the electrolyte becamebrittle and crumbly. Then, the electrolyte was immersed in water at 60°C. for 5 hours, so that the above aqueous solution was removed from thefluoroelectrolyte. Thereafter, treatment of immersing the electrolyteinto a 60° C. 2 N hydrochloric acid for 1 hour was repeated 5 times,with the hydrochloric acid exchanged at every treatment. Ion exchangewater was then put thereinto, the fluoropolymer electrolyte was left for5 hours such that the electrolyte was not collapsed, and the supernatantfluid was removed. This treatment was repeated until the pH of thedischarged water was 5 or higher. Also in this step, the fluoropolymerelectrolyte absorbed ion exchange water in an amount about 28.0 times aslarge as the dried weight of the fluoroelectrolyte precursor. Therefore,its volume remarkably increased. The fluoroelectrolyte treated asmentioned above was carefully recovered and then dried. Thereby, afluoropolymer electrolyte was obtained.

This fluoropolymer electrolyte was put into a 5-L autoclave togetherwith an ethanol aqueous solution (water: ethanol=50.0:50.0 (mass ratio))and the autoclave was hermetically sealed. The mixture was heated to160° C. under stirring with a wing, and was maintained for 5 hours.Then, the autoclave was naturally cooled down, and a uniformfluoropolymer electrolyte solution (viscosity: 400 mPa·s) having asolids concentration of 5% by mass was produced.

This fluoropolymer solution was concentrated at 80° C. under reducedpressure, and thereby a cast solution having a solids concentration of20% by mass was obtained. This cast solution was cast on atetrafluoroethylene film using a doctor blade. The solution waspre-dried in an oven at 60° C. for 30 minutes, and then dried at 80° C.for 30 minutes, so that the solvent was removed. The dried product wasfurther heated at 160° C. for 1 hour, and thereby a fluoropolymerelectrolyte membrane having a thickness of about 50 μm was obtained.

This fluoropolymer electrolyte membrane had an EW of 455, and a distancebetween ionic clusters of 2.3 nm. The ion conductivities were 0.10 S/cmat 110° C. and 25% RH and 0.20 S/cm at 110° C. and 50% RH. Further, the25° C. water content was 180%.

Except that the above fluoropolymer electrolyte membrane was used, anMEA was produced in the same manner as in the step (1.8) of Example 1,and the fuel cell evaluation was performed. As a result, the currentdensity was 0.57 A/cm² after the MEA was maintained at a voltage of 0.6V for 20 hours under the conditions of a cell temperature of 80° C. andthe 50° C. saturated vapor pressure (corresponding to humidity 26% RH).

Except that the above fluoropolymer electrolyte solution (0.825 g) andethanol (8.175 g) were used instead of the acid-type aqueous dispersionB, an electrode catalyst layer was produced in the same manner as in thestep (1.7) of Example 1. Except that this electrode catalyst layer wasused, an MEA was produced in the same manner as in the step (1.9) ofExample 1, and the fuel cell evaluation was performed. As a result, thecurrent density was 0.46 A/cm² after the MEA was maintained at a voltageof 0.6 V for 20 hours under the conditions of a cell temperature of 80°C. and the 50° C. saturated vapor pressure (corresponding to humidity26% RH). The obtained current density was not so high as that in Example1.

Comparative Example 2

A fluoroelectrolyte having repeating units derived from CF₂═CF₂ andCF₂═CF—O—(CF₂)₂—SO₃H with an EW of 720 was produced as follows.

A 189-L-capacity SUS316-made pressure-resistant container provided witha stirring wing and a temperature-controlling jacket was charged withwater purified by reverse osmosis membrane (90.5 kg), C₇F₁₅COONH₄ (0.945g), and CF₂═CFOCF₂CF₂SO₂F (5.68 kg). The air inside the system wasreplaced with nitrogen and then the container was evacuated.Subsequently, TFE was introduced therein until the internal pressurereached 0.2 MPaG. The temperature was controlled under stirring at 189rpm so that the internal temperature was 47° C., and CF₄ of 0.1 MPaG asan explosion inhibitor was introduced. Thereafter, TFE was additionallyintroduced so that the internal pressure was 0.70 MPaG. A solution of(NH₄)₂S₂O₈ (47 g) in water (3 L) was introduced into the system, andthereby polymerization was initiated. Thereafter, TFE was added so as tomaintain the internal pressure at 0.7 MPaG. For every 1 kg of TFE, 0.7kg of CF₂═CFOCF₂CF₂SO₂F was supplied so that the polymerization wascontinued.

After 360 minutes from the polymerization initiation, that is, at thetime when 24 kg in total of TFE was additionally introduced, the TFE wasreleased from the pressure and the polymerization was stopped. Theobtained polymerization liquid (140 kg) was mixed with water (200 kg),and further mixed with nitric acid to be coagulated. The coagulatedpolymer was centrifuged, and ion exchange water was flowed therethroughso that the polymer was washed. Thereafter, the polymer was dried at 90°C. for 24 hours and subsequently at 150° C. for 24 hours using a hot-airdrier. Thereby, 34 kg of a polymer was obtained.

A 28-kg portion of the polymer was rapidly charged into a 50-L Hastelloyvibrating reactor (OKAWARA MFG. CO., LTD.). The polymer was heated to100° C. while vibrated at a vibration number of 50 rpm under evacuation.Then, nitrogen was introduced therein until the gauge pressure reached−0.05 MPaG. Thereafter, a gaseous halogenating agent which was preparedby diluting F₂ gas to 20% by mass with nitrogen gas was introduced intothe reactor until the gauge pressure reached 0.0 MPaG, and the systemwas maintained for 30 minutes.

The gaseous halogenating agent inside the reactor was discharged and thereactor was evacuated. Then, a gaseous halogenating agent prepared bydiluting F₂ gas to 20% by mass with nitrogen gas was introduced thereinuntil the gauge pressure reached 0.0 MPaG, and the system was maintainedfor 3 hours.

The system was cooled down to room temperature and the gaseoushalogenating agent inside the reactor was discharged. After vacuum andnitrogen introduction were repeated 3 times, the reactor was released.Thereby, 28 kg of a polymer (fluoropolymer electrolyte) was obtained.

The obtained polymer had an MFR of 3.0 g/10 min, and contained 18 mol %of a repeating unit derived from the SO₃H group-containing monomer.

Except that this fluoropolymer electrolyte was used, a fluoropolymerelectrolyte solution and a fluoropolymer electrolyte membrane wereproduced and the EW, ion-cluster distance, and conductivity weremeasured in the same manner as in Comparative Example 1. As a result,the EW was 720 and the distance between ionic clusters was 3.1 nm. Theconductivity was 0.06 S/cm at 110° C. and 50% RH; that is, a desiredhigh conductivity was not achieved.

Except that the above fluoropolymer electrolyte membrane was used, anMEA was produced in the same manner as in the step (1.10) of Example 1,and the fuel cell evaluation was performed. As a result, the currentdensity was 0.25 A/cm² after the MEA was maintained at a voltage of 0.6V for 20 hours under the conditions of a cell temperature of 80° C. andthe 50° C. saturated vapor pressure (corresponding to humidity 26% RH);that is, a high current density was not achieved.

Except that the above fluoropolymer electrolyte solution was used, anelectrode catalyst layer and an MEA were produced in the same manner asin the step (1.11) of Example 1, and the fuel cell evaluation wasperformed. As a result, the current density was 0.25 A/cm² after the MEAwas maintained at a voltage of 0.6 V for 20 hours under the conditionsof a cell temperature of 80° C. and the 50° C. saturated vapor pressure(corresponding to humidity 26% RH); that is, a high current density wasnot achieved.

Example 2

Except that the amount of CF₂═CFOCF₂CF₂SO₂F was changed from 1,150 g to1,300 g and TFE was added in order to maintain the internal pressure at0.02 MPaG so that the polymerization was continued, a fluoropolymerelectrolyte emulsion having repeating units derived from CF₂═CF₂ andCF₂═CF—O—(CF₂)₂—SO₃H with an EW of 399 was produced in the same manneras in Example 1. After 16 hours from the polymerization initiation, thatis, at the time when 323 g in total of TFE was additionally introduced,the TFE was released from the pressure and the polymerization wasstopped. Thereby, 4,570 g of a polymerization liquid (precursoremulsion) was obtained.

The precursor polymer obtained in the same manner as in Example 1 had anMFR of 15.6 g/10 min. The fluoropolymer electrolyte emulsion contained45.2 mol % of a repeating unit derived from the SO₃H group-containingmonomer of the fluoropolymer electrolyte. The fluoropolymer electrolytehad an average particle size of 50 nm and an aspect ratio of 1.0. Theratio (the number of SO₂F groups)/(the number of SO₃Z groups) was 0.

Thereafter, a fluoropolymer electrolyte membrane and an electrodecatalyst layer were produced in the same manner as in Example 1. Thefluoropolymer electrolyte membrane had an EW of 399, a distance betweenionic clusters of 2.3 nm, and ion conductivities of 0.10 S/cm at 110° C.and 23% RH and 0.22 S/cm at 110° C. and 50% RH. Further, the 25° C.water content was 210%. The platinum content of the electrode catalystlayer was 0.17 mg/cm² in an anode electrode and 0.32 mg/cm² in a cathodeelectrode. In the fuel cell evaluation performed in the same manner asin the step (1.10) of Example 1 except that the above electrolytemembrane was used, the current density was 0.61 A/cm² after the MEA wasmaintained at a voltage of 0.6 V for 20 hours under the conditions of acell temperature of 80° C. and the 50° C. saturated vapor pressure(corresponding to humidity 26% RH). Further, in the fuel cell evaluationperformed in the same manner as in the step (1.11) of Example 1 exceptthat the above electrode catalyst layer was used, the current densitywas 0.51 A/cm² after the MEA was maintained at a voltage of 0.6 V for 20hours under the conditions of a cell temperature of 80° C. and the 50°C. saturated vapor pressure (corresponding to humidity 26% RH).

Example 3

Except that the internal temperature was controlled to 17.5° C. and TFEwas added in order to maintain the internal pressure at 0.09 MPaG sothat the polymerization was continued, a fluoroelectrolyte emulsionhaving repeating units derived from CF₂═CF₂ and CF₂═CF—O—(CF₂)₂—SO₃Hwith an EW of 470 (MFR of 1.8) was produced in the same manner as inExample 1. After 9 hours from the polymerization initiation, that is, atthe time when 401 g in total of TFE was additionally introduced, the TFEwas released from the pressure and the polymerization was stopped.Thereby, 4,664 g of a polymerization liquid (precursor emulsion) wasobtained.

The precursor polymer obtained in the same manner as in Example 1 had anMFR of 1.8 g/10 min.

The fluoropolymer electrolyte emulsion contained 34.2 mol % of arepeating unit derived from the SO₃H group-containing monomer of thefluoropolymer electrolyte. The fluoropolymer electrolyte had an averageparticle size of 35 nm and an aspect ratio of 1.0. The ratio (the numberof SO₂F groups)/(the number of SO₃Z groups) was 0.

Thereafter, except that the obtained fluoroelectrolyte emulsion wasused, a fluoropolymer electrolyte membrane and an electrode catalystlayer were produced in the same manner as in Example 1. Thefluoropolymer electrolyte membrane had an EW of 470, a distance betweenionic clusters of 2.3 nm, and ion conductivities of 0.10 S/cm at 110° C.and 26% RH and 0.18 S/cm at 110° C. and 50% RH. Further, the 25° C.water content was 150%. The platinum content of the electrode catalystlayer was 0.17 mg/cm² in an anode electrode and 0.32 mg/cm² in a cathodeelectrode. In the fuel cell evaluation performed in the same manner asin the step (1.10) of Example 1 except that the above electrolytemembrane was used, the current density was 0.54 A/cm² after the MEA wasmaintained at a voltage of 0.6 V for 20 hours under the conditions of acell temperature of 80° C. and the 50° C. saturated vapor pressure(corresponding to humidity 26% RH). Further, in the fuel cell evaluationperformed in the same manner as in the step (1.11) of the example exceptthat the above electrode catalyst layer was used, the current densitywas 0.48 A/cm² after the electrode catalyst layer was maintained at avoltage of 0.6 V for 20 hours under the conditions of a cell temperatureof 80° C. and the 50° C. saturated vapor pressure (corresponding tohumidity 26% RH).

Example 4

Except that the amount of C₇F₁₅COONH₄ was changed from 150 g to 60 g,the amount of CF₂═CFOCF₂CF₂SO₂F was changed from 1,150 g to 943 g, theinternal temperature was controlled to 38° C., and TFE was added inorder to maintain the internal pressure at 0.51 MPaG so that thepolymerization was continued, a fluoropolymer electrolyte emulsionhaving repeating units derived from CF₂═CF₂ and CF₂═CF—O—(CF₂)₂—SO₃Hwith an EW of 527 was produced in the same manner as in Example 1. After7 hours from the polymerization initiation, that is, at the time when381 g in total of TFE was additionally introduced, TFE was released fromthe pressure and the polymerization was stopped. Thereby, 4,260 g of apolymerization liquid (precursor emulsion) was obtained.

The precursor polymer obtained in the same manner as in Example 1 had anMFR of 16 g/10 min. The fluoropolymer electrolyte emulsion contained 29mol % of a repeating unit derived from the SO₃H group-containing monomerof the fluoropolymer electrolyte. The fluoropolymer electrolyte had anaverage particle size of 62 nm and an aspect ratio of 1.0. The ratio(the number of SO₂F groups)/(the number of SO₃Z groups) was 0.

Thereafter, except that the obtained fluoroelectrolyte emulsion wasused, a fluoropolymer electrolyte membrane and an electrode catalystlayer were produced in the same manner as in Example 1. Thefluoropolymer electrolyte membrane had an EW of 527, a distance betweenionic clusters of 2.4 nm, and ion conductivities of 0.10 S/cm at 110° C.and 30% RH and 0.14 S/cm at 110° C. and 50% RH. Further, the 25° C.water content was 100%. The platinum content of the electrode catalystlayer was 0.17 mg/cm² in an anode electrode and 0.32 mg/cm² in a cathodeelectrode. In the fuel cell evaluation performed in the same manner asin the step (1.10) of Example 1 except that the above electrolytemembrane was used, the current density was 0.51 A/cm² after theelectrolyte membrane was maintained at a voltage of 0.6 V for 20 hoursunder the conditions of a cell temperature of 80° C. and the 50° C.saturated vapor pressure (corresponding to humidity 26% RH). Further, inthe fuel cell evaluation performed in the same manner as in the step(1.11) of the example except that the above electrode catalyst layer wasused, the current density was 0.46 A/cm² after the electrode catalystlayer was maintained at a voltage of 0.6 V for 20 hours under theconditions of a cell temperature of 80° C. and the 50° C. saturatedvapor pressure (corresponding to humidity 26% RH).

Example 5

Except that the amount of C₇F₁₅COONH₄ was changed from 150 g to 60 g,the amount of CF₂═CFOCF₂CF₂SO₂F was changed from 1,150 g to 950 g, theinternal temperature was controlled to 38° C., and TFE was added inorder to maintain the internal pressure at 0.42 MPaG so that thepolymerization was continued, a fluoropolymer electrolyte emulsionhaving repeating units derived from CF₂═CF₂ and CF₂═CF—O—(CF₂)₂—SO₃Hwith an EW of 548 was produced in the same manner as in Example 1. After5 hours from the polymerization initiation, that is, at the time when381 g in total of TFE was additionally introduced, the TFE was releasedfrom the pressure and the polymerization was stopped. Thereby, 4,328 gof a polymerization liquid (precursor emulsion) was obtained.

The precursor polymer obtained in the same manner as in Example 1 had anMFR of 7.2 g/10 min.

The fluoropolymer electrolyte emulsion contained 27.0 mol % of arepeating unit derived from the SO₃H group-containing monomer of thefluoropolymer electrolyte. The fluoropolymer electrolyte had an averageparticle size of 47 nm and an aspect ratio of 1.0. The ratio (the numberof SO₂F groups)/(the number of SO₃Z groups) was 0.

Thereafter, except that the obtained fluoroelectrolyte emulsion wasused, a fluoropolymer electrolyte membrane and an electrode catalystlayer were produced in the same manner as in Example 1. Thefluoropolymer electrolyte membrane had an EW of 548, a distance betweenionic clusters of 2.4 nm, and ion conductivities of 0.10 S/cm at 110° C.and 35% RH and 0.13 S/cm at 110° C. and 50% RH. Further, the 25° C.water content was 90%. The platinum content of the electrode catalystlayer was 0.17 mg/cm² in an anode electrode and 0.32 mg/cm² in a cathodeelectrode. In the fuel cell evaluation performed in the same manner asin the step (1.10) of Example 1 except that the above electrolytemembrane was used, the current density was 0.49 A/cm² after theelectrolyte membrane was maintained at a voltage of 0.6 V for 20 hoursunder the conditions of a cell temperature of 80° C. and the 50° C.saturated vapor pressure (corresponding to humidity 26% RH). Further, inthe fuel cell evaluation performed in the same manner as in the step(1.11) of the example except that the above electrode catalyst layer wasused, the current density was 0.45 A/cm² after the electrode catalystlayer was maintained at a voltage of 0.6 V for 20 hours under theconditions of a cell temperature of 80° C. and the 50° C. saturatedvapor pressure (corresponding to humidity 26% RH).

Example 6

Except that the amount of C₇F₁₅COONH₄ was changed from 150 g to 60 g,the amount of CF₂═CFOCF₂CF₂SO₂F was changed from 1,150 g to 958 g, theinternal temperature was controlled to 38° C., and TFE was added inorder to maintain the internal pressure at 0.53 MPaG so that thepolymerization was continued, a fluoropolymer electrolyte emulsionhaving repeating units derived from CF₂═CF₂ and CF₂═CF—O—(CF₂)₂—SO₃Hwith an EW of 579 was produced in the same manner as in Example 1. After5 hours from the polymerization initiation, that is, at the time when383 g in total of TFE was additionally introduced, the TFE was releasedfrom the pressure and the polymerization was stopped. Thereby, 4,343 gof a polymerization liquid (precursor emulsion) was obtained.

The precursor polymer obtained in the same manner as in Example 1 had anMFR of 2.8 g/10 min.

The fluoropolymer electrolyte emulsion contained 24.9 mol % of arepeating unit derived from the SO₃H group-containing monomer of thefluoropolymer electrolyte. The fluoropolymer electrolyte had an averageparticle size of 78 nm and an aspect ratio of 1.0. The ratio (the numberof SO₂F groups)/(the number of SO₃Z groups) was 0.

Thereafter, except that the obtained fluoroelectrolyte emulsion wasused, a fluoropolymer electrolyte membrane and an electrode catalystlayer were produced in the same manner as in Example 1. Thefluoropolymer electrolyte membrane had an EW of 579, a distance betweenionic clusters of 2.5 nm, and ion conductivities of 0.10 S/cm at 110° C.and 40% RH and 0.12 S/cm at 110° C. and 50% RH. Further, the 25° C.water content was 80%. The platinum content in the electrode catalystlayer was 0.17 mg/cm² in an anode electrode and 0.32 mg/cm² in a cathodeelectrode. In the fuel cell evaluation performed in the same manner asin the step (1.10) of Example 1 except that the above electrolytemembrane was used, the current density was 0.47 A/cm² after theelectrolyte membrane was maintained at a voltage of 0.6 V for 20 hoursunder the conditions of a cell temperature of 80° C. and the 50° C.saturated vapor pressure (corresponding to humidity 26% RH). Further, inthe fuel cell evaluation performed in the same manner as in the step(1.11) of the example except that the above electrode catalyst layer wasused, the current density was 0.42 A/cm² after the electrode catalystlayer was maintained at a voltage of 0.6 V for 20 hours under theconditions of a cell temperature of 80° C. and the 50° C. saturatedvapor pressure (corresponding to humidity 26% RH).

Comparative Example 3

A fluoroelectrolyte having repeating units derived from CF₂═CF₂ andCF₂═CF—O—(CF₂)₂—SO₃H with an EW of 705 was produced as follows.

A 6-L-capacity SUS316-made pressure-resistant container provided with astirring wing and a temperature-controlling jacket was charged withwater purified by reverse osmosis membrane (2,950 g), C₇F₁₅COONH₄ (60g), and CF₂═CFOCF₂CF₂SO₂F (180 g). The air inside the system wasreplaced with nitrogen, and then the container was evacuated.Subsequently, TFE was introduced therein until the internal pressurereached 0.2 MPaG. The temperature was controlled so that the internaltemperature was 48° C. under stirring at 400 rpm, and CF₄ of 0.1 MPaG asan explosion inhibitor was introduced into the container. TFE wasadditionally introduced so that the internal pressure was 0.70 MPaG.Then, a solution of (NH₄)₂S₂O₈ (1.5 g) in water (20 g) was injectedtherein, so that polymerization was initiated. Thereafter, TFE was addedso as to maintain the internal pressure at 0.70 MPaG. For every 10 g ofTFE, 6.5 g of CF₂═CFOCF₂CF₂SO₂F was added to continue thepolymerization.

After 218 minutes from the polymerization initiation, that is, at thetime when 774 g of TFE was additionally introduced, the TFE was releasedfrom the pressure and the polymerization was stopped. A 4,400-g portionof the obtained polymerization liquid (precursor emulsion) was mixedwith water (4,400 g), and then mixed with nitric acid to be coagulated.The coagulated polymer was filtered. Re-dispersion in water andfiltration was repeated 3 times. Then, the polymer was dried at 90° C.for 12 hours and subsequently at 120° C. for 12 hours using a hot-airdrier. Thereby, 1,200 g of a polymer was obtained.

The obtained polymer (fluoropolymer electrolyte precursor) had an MFR of3.5 g/10 min and contained 19 mol % of a repeating unit derived from theSO₃H group-containing monomer.

Except that this polymer (fluoropolymer electrolyte precursor) was used,a fluoropolymer electrolyte solution and a fluoropolymer electrolytemembrane were produced and the EW, ion-cluster distance, andconductivity were measured in the same manner as in ComparativeExample 1. As a result, the EW was 705 and the distance between ionicclusters was 2.7 nm. The conductivity was 0.08 S/cm at 110° C. and 50%RH; that is, a desired high conductivity was not achieved.

Except that the above fluoropolymer electrolyte membrane was used, anMEA was produced in the same manner as in the step (1.10) of Example 1,and the fuel cell evaluation was performed. As a result, the currentdensity was 0.27 A/cm² after the MEA was maintained at a voltage of 0.6V for 20 hours under the conditions of a cell temperature of 80° C. andthe 50° C. saturated vapor pressure (corresponding to humidity 26% RH);that is, a high current density was not achieved.

Except that the fluoropolymer electrolyte solution was used, anelectrode catalyst layer and an MEA were produced in the same manner asin the step (1.11) of Example 1, and the fuel cell evaluation wasperformed. As a result, the current density was 0.26 A/cm² after the MEAwas maintained at a voltage of 0.6 V for 20 hours under the conditionsof a cell temperature of 80° C. and the 50° C. saturated vapor pressure(corresponding to humidity 26% RH); that is, a high current density wasnot achieved.

Comparative Example 4 4.1

A 3,000-ml-capacity stainless-steel stirring autoclave was charged witha 10% aqueous solution of C₇F₁₅COONH₄ (300 g) and pure water (1,170 g).The autoclave was sufficiently evacuated and the inside air was replacedwith nitrogen, and the autoclave was then sufficiently evacuated.Tetrafluoroethylene [TFE] gas was introduced therein so that the gaugepressure was 0.2 MPa, and the autoclave was heated up to 50° C.Thereafter, CF₂═CFOCF₂CF₂SO₂F (100 g) was charged, and TFE gas wasintroduced therein so that the gauge pressure was raised to 0.7 MPa. Asolution of ammonium persulfate [APS] (0.5 g) in pure water (60 g) wascharged therein and polymerization was initiated.

In order to supply TFE consumed during the polymerization, TFE wascontinuously supplied to the autoclave so that the pressure wasmaintained at 0.7 MPa. CF₂═CFOCF₂CF₂SO₂F in an amount corresponding to0.53 times in mass ratio larger than that of the further supplied TFEwas continuously supplied so that the polymerization was continued. Atthe time when 522 g in total of TFE was supplied, the pressure insidethe autoclave was released and the polymerization was stopped.Thereafter, the autoclave was cooled down to room temperature. Thereby,2,450 g of a slightly opaque precursor emulsion containing about 33% bymass of the fluoropolymer electrolyte precursor was obtained.

Part of the precursor emulsion was taken out and coagulated with nitricacid. Then, the coagulated product was washed with water and dried. Thehigh-temperature NMR measurement showed that the fluoropolymer precursorcontained 19 mol % of the fluorovinyl ether derivative units.

4.2

The precursor emulsion (50 ml) obtained in the step (4.1) was diluted5-fold with pure water, and was stirred in a 500-ml-capacity beaker andthe temperature was raised to 55° C. The pH was maintained at 10 orhigher while 10% by mass of a sodium hydroxide aqueous solution wasdropwise added, and —SO₂F of the fluoropolymer precursor was hydrolyzed.Although reduction in the pH was no longer observed after about 3-hourhydrolysis, the hydrolysis was further continued for another 2 hours,and then stopped. No deposition of the fluoropolymer was visuallyobserved during the hydrolysis.

4.3

The reaction solution obtained in the step (4.2) was mixed with 1 Nhydrochloric acid, and thereby hydrolysis with acid was performed.Low-molecular-weight matter was removed and the fluoropolymer waspurified and concentrated by centrifugal ultrafiltration usingCentriprep YM-10 (Amicon). The obtained fluoroelectrolyte emulsion had aconcentration of the fluoropolymer electrolyte of 43% by mass, andcontained a fluoropolymer electrolyte having stable —SO₃K.

The fluoroelectrolyte emulsion was diluted 100-fold with pure water anddripped onto an aluminum plate. Then, the dripped emulsion was dried at60° C. so that a sample for particle-shape measurement was prepared. Thesample was measured using an atomic force microscope [AFM], and 20particles within the obtained image were randomly selected out. Theaspect ratio was 1.0 and the average particle size was 100 nm.

4.4

The fluoroelectrolyte emulsion obtained in the step (4.3) was mixed withan ethanol-isopropanol (1:1 in volume) mixed solution in a volume halfof the emulsion, and thereby a dispersion composition for thin filmformation was obtained. The dispersion composition for thin filmformation was applied to a glass plate, and then dried at roomtemperature, so that a colorless transparent membrane was obtained. Theobtained membrane was heated at 300° C. for 10 minutes, and thusimmobilized, and then immersed in pure water so that the membrane wasseparated from the glass plate. Thereby, an electrolyte membrane wasobtained. The obtained electrolyte membrane had a thickness of 12 to 17μm. The EW measurement was performed on the obtained electrolytemembrane, and the EW was 705.

The obtained polymer (fluoropolymer electrolyte precursor) had an MFR of3.5 g/10 min and contained 19 mol % of a repeating unit derived from theSO₃H group-containing monomer.

Except that this polymer (fluoropolymer electrolyte precursor) was used,a fluoropolymer electrolyte solution and a fluoropolymer electrolytemembrane were produced and the EW, ion-cluster distance, andconductivity were measured in the same manner as in ComparativeExample 1. As a result, the EW was 705 and the distance between ionicclusters was 2.7 nm. The conductivity was 0.08 S/cm at 110° C. and 50%RH; that is, a desired high conductivity was not achieved.

INDUSTRIAL APPLICABILITY

The electrolyte emulsion of the present invention enables to easilyprovide a fuel cell having high performance even under high-temperatureand low-humidity conditions at low cost. The electrolyte emulsion of thepresent invention may be used for various fuel cells including directmethanol-type fuel cells, electrolysis of water, electrolysis ofhalogenated hydroacids, electrolysis of common salt, oxygenconcentrators, humidity sensors, and gas sensors.

1. A method for producing an electrolyte emulsion, the methodcomprising: Step (1) in which an ethylenic fluoromonomer and afluorovinyl compound having an SO₂Z¹ group, wherein Z¹ is a halogenelement, are copolymerized at a polymerization temperature of 0° C. orhigher and 40° C. or lower to provide a precursor emulsion containing afluoropolymer electrolyte precursor; and Step (2) in which a basicreactive liquid is added to the precursor emulsion and the fluoropolymerelectrolyte precursor is chemically treated, whereby an electrolyteemulsion with a fluoropolymer electrolyte dispersed therein is provided,wherein the electrolyte emulsion has an equivalent weight (EW) of 250 ormore and 700 or less.
 2. The method for producing an electrolyteemulsion according to claim 1, the electrolyte emulsion comprising: anaqueous medium; and a fluoropolymer electrolyte dispersed in the aqueousmedium, the fluoropolymer electrolyte having a monomer unit that has anSO₃Z group, wherein Z is an alkali metal, an alkaline-earth metal,hydrogen, or NR¹R²R³R⁴, wherein R¹, R², R³, and R⁴ each are individuallya C1-C3 alkyl group or hydrogen, the electrolyte having an equivalentweight (EW) of 250 or more and 700 or less, a proton conductivity at110° C. and relative humidity 50% RH of 0.10 S/cm or higher, and a ratio(the number of SO₂F groups)/(the number of SO₃Z groups) of 0 to 0.01,and the electrolyte being a spherical particulate substance having anaverage particle size of 10 to 500 nm is produced.